If you are fusing two layers
If you are fusing two layers of 3 mm glass, then just 1/4″ or so larger all the way around should do it. Anything thicker needs larger paper or to be dammed so it doesn’t spread over the edges of the paper. You can hand sand the hazing off and refire.
Make sure you are using Bullseye Thinfire or Spectrum Papyros, and not the “cheap stuff”. It will curl every time. You can use pieces of scrap glass to weight down the edges of paper so it doesn’t curl.
Jester’s Baubles Fused Glass Designs