One thing I find the most helpful is to trim as I go. If I'm working with hst, for example, I make the hst, then I trim to the size it needs to be (say 4.5") before I add it into the bigger block design. I find I lose fewer points that way and my finished block size is closer to what it is supposed to be. If my finished block is a little too small, sometimes I can press it into submission and get it a little bigger. If it's a little too big, you can sometimes ease them together and they'll be okay. Put the piece/block with the most bulk on the bottom when you're sewing. If it's way too big, I take it apart and resew or throw it in the orphan block box and redo completely. Clear as mud?