Plywood/MDF sheets/etc ships in a standard size of 96“ x 48”. You can buy smaller sheets for testing, but it's usually a premium charge. We've put together a set of PDFs for plywood sheets for the new 32×18 laser bed.
This is a trivial task if you have a 12×24 laser. Just have the sheet ripped down to 12×24 pieces and take them home. This is what we usually do for prototyping.
All of these layouts are zero waste layouts. The goal is to keep waste to a minimum!
12×24 - just in case you wanted it.
16×24 - Slightly larger for some projects
16×32 - The largest zero waste cut we could come up with.