I made it over 15 years ago but I cut the snowball blocks 8" and the twisted blocks were cut 3" making a 7.5" finished square. You subtract 1/2" from the snowball block and that creates a 7.5" finished block. Sew the 3" twisted blocks and that also creates a 7.5" block. Is that making sense?? I know I did a snowball and not a pieced triangle so I think those were also 3" squares. That is a guess.
I am not good with math anymore having being disable. But I would make a test block and see how things work out. Cut a 15.5 snowball block and 5.5" pieces for the Twisted block. Hope that is correct. This is NOT an easy pattern.