If you lay the HSTs on the corner of a ruler, you will get the unfinished size of the block that the HSTs can work with. If you want to find out what size the original square would have been, add 3/8".
Here is an example: If the measurement of the side of the HST is 3 1/2", then the square that it was cut from would have been 3 7/8" (this would be the number referenced in a pattern book). The finished size of any block that is made from them would be 6".
Hope this helps.