Here is the link for the half-square triangle tutorial.
http://www.wetcanvas.com/forums/showthread.php?t=590241
A 6" square will give you 2 1/2" HSTs, an 8" square will give you 3 1/2" HSTs, and a 10" square will give you 4 1/2" HSTs.
Make sure you sew 1/4" from the diagonal lines only!