big or small block

is it true that 351 cleveland engines are small block, and 351 windsor engines are big block?

no

they are both small blocks.

Ford "Smallblock"/"Big Block" is a strange subject. Technically the only smallblocks are the 289/302/351W family, however most of the blocks have lots of differences. The 352/360/390/428 are called "FE" blocks but were pretty much a big block. The 351C is something entirely in itself, it is a large canted-valve headed smallblock with Windsor bolt pattern and motor mounts. 351M/400 used the same style intake/heads but the blocks were a lot different with bigger mains and 429/460 Bolt pattern/motor mounts. Many consider these to be Big Blocks, even though they are very similar externally to a 351C. The true "BBF" is the 429/460.