If it is a plain square that measures 2" finished, then the square is cut at 2 1/2" (1/4 seam allowance on all sides - helps to draw a 2" square then add the 1/4" seam allowance on all sides then measure - I'm a visual person).
If making a half square triangle, you need to add an additional 3/8" for a total of 2 7/8". The diagonal cut takes up the additional 3/8".
I like the tutorial in the link that I posted above it shows how to do this. Also, if you have a problem with accurate seams, just make the squares at 3", put them right sides together, draw a diagonal line on the top one, and stitch 1/4" from the drawn line, the cut on the line, then square to 2 1/2" unfinished size.