Sharon Schamber has some videos on Youtube that show how to square up a block using starch and pins. There's only so much you can do with an 8-point star to square up because you absolutely must leave enough seam allowance so the points are not chopped off.
If the block is too big, even after blocking, you may have to ease the block edges.
If the block is too small, you may be able to "float" the block in order to preserve the star points. A basic way to make this work is to have sashing the same as the background color of the blocks. Here is an example:
http://www.bamrobot.com/pz-inch-7-su...35Ktzkjpg.html