If your blocks are just a little off (1/4" or less) you can often block them up to size. I use a cork bulletin board. For a 12 1/2" unfinished I took a permanent sharpy marker and drew a perfect 12 1/2" square on my cork board (used my square ruler). I then pin the block to the board matching up to the drawn lines. First I pin opposing corners (diagonal opposing corners) then work my way around the block pinning and stretching the block to fit. I use however many pins are necessary to get the block strectched out to size. Sometimes this can take up to 10 pins per side. Once pinned I take a spray mister bottle with just plain water and spray the block until it is wet. Leave it to air dry. I usually leave it overnight. This blocking procedure will often get your pieced blocks up to the correct size and square them up at the same time. When you remove all the pins the block will stay the size it was stretched to. If your blocks are smaller than 12 1/2" unfinished you may not be able to get as much as a 1/4" out of them. You will know once you start pinning them to the board if they won't make it at all.
I prefer this method to cutting down when points are involved. If the block is all squares and rectangles with no points, like a 9 patch or 4 patch, cutting down to smallest size may be more desireable.