Their website says the blocks finish at 4 1/2", but I am not sure if they mean after trimming to size, or after putting into the quilt. Whether the HSTs are 1 1/2" will depend on that. You have the instructions so will know this.
If interested in trying this method, using only 1/8" seams, you may be able to make 16 HSTs of 1 1/2" from 2 - 5" squares, but there should not be any trimming. Hope you can follow the chart - I would test it on scrap fabric first.
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