You find the smallest block. Trim the rest to that same size. You can use any square size ruler. I would trim based on how close the applique is to the edge. I am assuming all the butterflies are the same size, and the blocks are squares not rectangles and it is just the backing squares that are measuring different. Anything off 1/8" or less I would not trim down. Just remember it is slightly smaller like put a pin on it with a scrap of paper with the measurement.
For blocks with the butterfly close to the edge, I am assuming the opposite edge has ample room for trimming. My main concern is to ensure you leave yourself a seam allowance when trimming.
Try googling how to square up a quilt block. You will find many blogs and youtube videos.
Here is one I selected randomly.
http://www.craftsy.com/blog/2014/11/...a-quilt-block/
It isn't that hard. Do you have a square ruler that is bigger than the blocks? That should be all you need.