I always make mine larger then trim.
If you want to make alot at one time without using paper patterns or gadgets, here is what I do.
Use two large squares one light one dark right sides togather, draw an X and sew 1/4 in down each side of lines. Cut the square in half both ways then on the drawn lines. You will have 8 HSTs.
Here is the formula for the size sqs to cut: For 2 1/2 in triangles I use a 6 1/2 in sq ruler, for each additional size just add 2 inchs ( 3 1/2 use and 8 1/2 in square) etc.
To save even more time invest in a Quilt In A Day triangle square up ruler. You only make one cut to square up your triangle. It is so easy to just place the ruler line for the size you are making on the unopened triangle sewing line and make one cut, all squared up.