Will paper size has no effect on the validity of a will. In the past, legal size paper was a tradition, but either letter or legal size paper can be used.
On the other hand, the quality of the will paper is important. Ever notice the way that a document printed on copy paper discolors and deteriorates after a few years? Documents such as wills, trusts, deeds—which must last for decades—call for archival quality paper. Archival paper also suits the import of such documents.
Archival paper is acid free and made with cotton. The best is 100% cotton although 50% and 25% grades also resist discoloration and deterioration. A 24 lb. paper tends to stand up better than a 20 lb. A smoother, harder surface, such as a ledger finish, resists accumulation of dust and dirt better than softer bond, linen and laid finishes. Copy papers are usually 20 lb. and contain no cotton content.