Edit: Found it right next to the price.
Does everything in the Proofgrade store come in a roughly 12"x20" size? I noticed in my starter kit some of the pieces are around a quarter of that size, and the grain on e.g. the hardwood items makes them look like the smaller size.
I think the acrylic, draftboard, and plywood is all 12x20. The hardwood and leather is smaller, 6x12. The size is displayed just to the left of the price.
Go to the Shop and you can see the sizes of everything. Some things, like Thin Leather, are only available in 5 x 10" size.
Hardwoods are 12x6, as indicated on each page. Some materials come in multiple sizes, also indicated: