I like the 700mm x 300mm size. You can find a few on novelkeys at the moment, and search for them on amazon.
https://novelkeys.xyz/collections/deskpads/products/sour-x-nk_-deskpads
https://novelkeys.xyz/collections/deskpads/products/modern-life-deskpads?variant=30372032118877
but these aren’t any firmer. I think you can get some harder ones on amazon