I would square up to the size they should be according to the pattern you are following, if its a little on the small side you can do a bit of tweaking and ironing then check again on your mat to see if its square...thats what I do, and be careful before cutting off any apparently bigger sides as you may end up with an unsymetrical block :shock: Make sure you have enough seam allowance so as not to end up with no points on stars etc.
Check and double check is my motto BEFORE trimming :D