If you mean that your squares are cut to 2 1/4" (unfinished size) then that means they're 1 3/4" finished. To make HST's that finish at 1 3/4" you would add 7/8", meaning that you would cut the squares for HSTs at 2 5/8", just as your pattern says. If your HSTs are coming out larger then they should be trimmed. But you didn't say how much larger they are - if it's just a smidgeon then you're doing them right and you'll just be trimming a thread or two. Usually people have the opposite problem and their HSTs are too small. I would start measuring the HSTs and the squares and see if the squares are cut to the right size too. Both should be measuring 2 1/4" before you join them. It's much better to trim them before sewing them together, provided you can trim to the correct size for the pattern. Of course if your pattern only has these HSTs and squares, then you can make them any size and it will come out right, just not the same size as the pattern.