I make cardstock hexies with a Fiskars punch. I don't really trim my fabric to any specific shape; I'm using scraps so I rummage around for pieces big enough, pin my hexie to the middle-ish, do some test folds to make sure it's going to have enough seam allowance all the way around, and if there's too much extra I just lop off the extra. I'm not neat in my cutting at all, I don't care if there's extra fabric on the back and I don't cut to any particular shape. Just "big enough" is all I'm worried about.