But you could easily make a 12.5" FINISHED block (13" unfinished), as that works well on a 5x5 grid.
edited to correct - thanks Dunster (too early in the morning)
Each piece would FINISH at 2.5".
So the squares would all be cut at 3".
For the HST's blocks, the 3.5" starting squares are still big enough to trim down to 3" UNFINISHED.
And you're good to go.