I found a small cut-n-paste error in nettle.texinfo. SHA256_DIGEST_SIZE should be 32, not 20. This agrees with the text a few lines above the patch.
/ceder
--- nettle.texinfo~ 2007-03-03 10:16:46.000000000 +0100 +++ nettle.texinfo 2007-03-03 10:17:23.000000000 +0100 @@ -510,7 +510,7 @@ @end deftp
@defvr Constant SHA256_DIGEST_SIZE -The size of an SHA256 digest, i.e. 20. +The size of an SHA256 digest, i.e. 32. @end defvr
@defvr Constant SHA256_DATA_SIZE