I like those blocks! Those colors just glow!
What "finished" size do you want for the mini-pinwheels?
Theoretically speaking, if one makes HST units from squares, one cuts the square 7/8 inch larger than the size you want the finished unit.
Example: If you want a 2 inch HST unit, cut the squares 2-7/8 inch.
HOWEVER, many of us have much better results if we cut the square one inch larger than the result wanted.
Example: Cut two three inch squares to finish to two inch HST units and trim.
If you wanted a 3 inch "finished size" mini-pinwheel - Cut the squares 2.5 inches for the HST units - trim the units to 2 inches - then each HST will "finish" at 1.5 inches -
You already know that it takes four of the HST units to make a pinwheel.