here is good tutorial,with a little video,from quilt university:

http://www.quiltuniversity.com/squaring_up.htm

i always square up my blocks before i start sewing them together for a quilt. i learned the hard way,when my my rows were off! lol

Mine is 12 1/2 X 12 1/2, I have never made any blocks bigger
than this.

I have a 16 by 16 Quilt in a Day ruler I picked up from a trunk show, I don't remember it being very expensive either. You can purchase one on Eleanor Burns Quilt in a Day website, and I use it quite frequently. I think 16" is the largest I have made a block to date also. Lol*

Thanks so much. I often wondered how to square up without making the block uneven design wise.

I have a 20 1/2 square and use it for lots of different cuts.

If I could have only one, I would start with the 12.5 inch one but if you work on larger blocks, the 16.5" one is very useful. I also use my 6.5 inch one a lot but the 12.5 inche one does everything the smaller one does, so I still would start with the 12.5" square as my first one.

12 1/2....but I'm in the market for even larger...Its great for squaring! I just don't care for the 12.5 one that I have..the numbers and markings are hard to read. (I know it couldn't be my eyes!! :wink::)

12.5

I have a 20 and my smallest is a 2.5 and all the ones in between ,use them all at one time or the other. lol

All the way up to 20-1/2 sq. I use this one a lot for applique blocks.