For my shelter just about any size will do. They have some cages that are big and some that are small. I think on a thread further back the recomended sizes for cat cages were 12 x 12 and
24 x 24. I made some that were 18 x 18 and they were fine. I made some long and narrow like maybe 12 x 20 and they worked good too. I really don't think it matters because they use anything from old sheets and towels to small donated fur beds. The only thing that really matters is that they are washable!!
You should have your hands full if you're cutting up five ocmforters! That will make quit a few. I got about 10 from an old queen size mattress pad. All I did with that was just cut it evenly across and down and just used whatever size they came out to be.

Your wonderful for putting all this work into them.