Originally Posted by MTS

Here's my HST primer.

The exact calculation for making HST's using this method (right sides together, draw diagonal, sew 1/4" each side of line, cut on line) is as follows:

Desired size of FINISHED HST plus 7/8" = cut size of beginning squares.

Well, working with 7/8" is a PITA, so I always add at least 1.25" to
the finished desired size, make the HST, press AND THEN trim it down to the exact size.

Now, in you case, if you're starting with 5" squares (charm pack?), then the largest finished HST you're going to get is 4". However, working with a finished 3.75" is much easier.

This means the smaller HST's will be 1.25" finished. You could cut the starting squares 2.25" .

Those are kind of small, so you might want to rethink your proportions.
Each block in that pattern - the large HST bordered by smaller HST on four sides - would be just 6.25 finished.

So you might want to make the proportions bigger all the way around as that requires a lot of HST's. A lot.

Hope that was helpful.