I've often thought it would helpful if the thing I was cooking on was close to as wide as the oven itself. In Australia ovens are usually 60x90cm. I often see and use American recipes because they're so common on the English speaking web and they quite often refer to sheet pans or baking sheets, which seem not to be a very common thing here. They look bigger than the types of things I can commonly buy here, which tend to be cookie pans that are really small. I used to think those American baking sheets were literally as big as the oven and slid in as racks but on further research it seems they're not actually that big and also need to sit on racks themselves and aren't as wide as the typical American home ovens.
I guess my theoretical baking rack would need it's rims to be less wide than the distance between rack grooves otherwise the food would touch the oven walls and baked goods that rise would might rise up to those grooves which would be no good either, but still that should only be a few cm. I actually sort of already have what I want as it came with the oven. It's a rack, that's not a wire and is a solid continuous sheet of metal that slides in to rack positions. The problem is, it always produced weird results when baking and seems to burn the bottoms of cookies and also has a large shallow ramp at the front that messes up what you can put on it. I read my oven instructions and discovered you're not actually supposed to cook on this thing and it's for catching drips. That's super weird to me since on occasion it's been used for this purpose accidentally and it's singularly unsuited to the task as any drips immediately bake right on to it and are impossible to remove and produce lots of smoke on the next use of the oven. I guess it's sort of better than nothing since I can theoretically clean that off when I take the rack out to clean it as opposed to the oven floor, but it's only marginally better since the effect of the baked on drippings is so thorough that it's near impossible to scrub off. Anyway, point is, while it's for whatever reason unsuited to the task presumably because of whatever it's made of and it's slightly odd shape, it's proof in my mind that the concept makes sense and can be done, and yet I can't find anything designed for this.
You can buy additional wire style racks, but seemingly not continuous metals sheets of appropriate size to fit in to the rack grooves.
You wouldn't want a baking sheet that matched the oven that closely.
You'd block all air movement, and reduce heat transfer as well. With most ovens, that means burnt bottoms and badly cooked tops. You might as well just throw the piece of metal on the range top and do it that way.
The drip tray (idk if there's another name) is meant to catch the stuff and have it on that instead of the bottom of the oven itself, which is much harder to clean than the removable drip tray. It's burning things because it's too thin. The heat doesn't get transferred evenly across the surface, and is too intense because of that. You need a little more mass than drip trays offer, though not as much as you would for pans on a hob.
Cleaning drip trays can be a pain in the ass, but usually you can either use harsher chemicals, or freely scrub with heavy abrasives. Tbh though, barkeepers friend should get the job done. A scraper can help too.
Baking much of anything on a rack isn't going to end well without adjusting the cook temp and time, if it even works at all. You can usually do stuff that's already partly cooked, or is solid enough it doesn't matter (like potatoes, rather than pastries and such). You wouldn't do anything that's a dough on the rack itself.
There are baking sheets that fit in slots in ovens, but they aren't the typical home oven. That's more bakery oriented ovens. Which, if you've got the funds and space, there's nothing stopping a person from having one at home, but I can't imagine justifying the cost and space for even high volume home baking. You make enough cookies at once to merit that kind of gear, you're turning out dozens at once.
Half sheet pans are generally your best bet tbh. You get better results, especially if you've got a convection oven. But you can get full sheet pans.
The general sizing is full sheet = a little over two feet × a foot and a half. (26×18 inches)
Half sheet is a foot and a half × a little over a foot. (18×13).
There are minor variations in those sizes, but that's the norm afaik. That's imperial units because it's late here and I ain't putzing with conversion lol.
A full sheet pan covers most of a rack in my oven, with a little room from front to back left over, and maybe two inches on the sides, but I never really use the one I have any more. I get better results with two half sheets on two different racks if I want to do bigger batches (but that's with convection).
Seriously, for a home oven, a sheet pan that fit into the rack slots would be a horrible idea. You'd have to reconfigure every recipe you tried because there just wouldn't be enough air flow around the edges.
Fwiw, if you don't like the drip tray that came with your oven, you can get super cheap aluminum and steel full sheet pans and use them on a lower rack. They tend to be a little easier to clean, and can usually be put in a dishwasher in a pinch.
For sourcing pans, try restaurant supply places. They tend to have the standard sized pans, which are supposed to be standard in Australia too (last time this general subject came up on reddit), if not necessarily what's marketed to home use.
Hadn't thought about the idea of it blocking airflow. The oven does have one big fan that at the back that spans most of the oven height that I'd hope would be moving air at all rack positions. That one activates when the oven is on regardless of fan or convection setting and then there's also an additional fan on top activated in fan forced mode and some type of vents or perforations on the sides which I assume move air around as well because there's a mode for cooking from the bottom top and sides at once.
I think I definitely do need a proper sheet pan though whether it's full oven sized or not. I don't know if it's just the world of internet cookery giving me the wrong impression, but they seem like standard equipment in the states that you can presumably just get anywhere. They're all small, thin, Teflon, high walled roasting trays or muffin tins here. They never give the desired results. There's the odd flat tray with shallow walls but they always seem very thin and nothing like those big heavy duty looking "baking sheets" the internet has me coveting. Wonder why they're not more common here.
Shall have to try a restaurant supply shop as you say. Can you typically buy just one or two of something from them? I thought they'd have minimum orders in the tens or something.
Yeah, baking sheets here are typically available singly at restaurant supply places. The ones I've been to are basically just stacks of things. You pull off however many you want and head to the register. All the restaurants I worked at (which was only a handful, and it wasn't my main job, so not a huge data set) were established, so you'd just be replacing stuff that was no longer usable, which meant whoever got sent was only picking up a few things, and sometimes only one thing.
Can't promise a brick & mortar place in Australia would be the same, but restaurants only need to buy in bulk when they're starting up, so it seems strange that they wouldn't sell things individually.
As far as thickness goes, they don't have to be much thicker than the drip tray you've got. A millimeter difference in thickness is a lot for this kind of thing. I can't recall how thick mine are, but it's still about twice as thick as my drip tray. Now, the fancy non stick calphalon type gear is much thicker than a drip tray. But you get too thick, and you run into trouble with the bottoms not browning, so there's an upper limit.
I second the restaraunt supply store route, I dont shop anywhere else for my kitchen.