I also have a variety of baking sheets and not all the same "size" are shaped the same way and I have to get creative to 'nest' them all in their storage space.
But to the OP's issue, how about a
Wilton baking sheet? It's stated dimensions are 17.25" x 11.5" but I think those are the
inside dimensions. I inherited one and it's sometimes a go-to since it's bigger than a quarter-sheet and (just a tad) smaller than a half-sheet and is still (relatively) non-stick after all these years. Sadly, the
outside measurements are 19.5"x 12.5" on mine so it might not work for you. I think
Target sells them too so you might be able to measure one in-store before buying.
You'd think appliance manufacturers would have a clue on standard size cookware. Now if the car designers would only find a place for my purse!