Typically
.12 (2.0CC)-.20 (3.3CC) is small block
.21 (3.5cc) and up to .36 (5.9CC) is big block
.37 and up is kind of a specialty category, mounting is typically different than the standard sized for big blocks.
as usual in RC there are exceptions, like the O.S. .21TM, its a big block internals...