Check your 1/4 inch seam. If you are making all of your blocks yourself it doesn't matter if the block size isn't what it's suppose to be. They will all go together and look just fine. The only time you want to make sure you have the 'correct' size is when 'it matters' ie: swaps, samplers (where each block may turn out a different size) and when you just WANT it the 'right' size.
As far as rulers large enough to square up a quilt block there are several out there. Omni, Creative Grid, June Taylor, Eleanor Burns all have large square rulers.
http://www.quiltbug.com/notions/rulers.htm
http://www.cottonpatch.co.uk/acatalo...re_Rulers.html
http://www.quiltinaday.com/shoponline/rulers.asp
http://www.ericas.com/quilting/tools/grids.htm
are just a few sites that have different types.