Susiequilter is right, if you're making half square triangles. To make a HST, you have to start with squares that are 7/8 of an inch larger than the sides of the triangle that you want. So, if you want to get a 2.5 inch block from two HST's, you will need to start with 3.375 inch squares rather than 3-inch squares.
However, if you were so lucky as to have all the other parts of the block come together evenly after the mishap with the HST's, then I agree with everyone else in that a couple of inches in length and/or width won't matter in the long run.