I guess I don't understand your question -- nor nativetexan's answer. If you're asking what size the HSTs are --- you can just use the ruler --- if they measure 3" square, then it's a 3" HST. If you're asking what size quilt all those will make, then add up how many you have. If you have 30 --- you can lay 5 across and 6 down....that takes 30 HST.....multiply 5 x 3" -- you get 15". Now subtract the 1/2" per block for the seam allowances. That adds up to 2" --- so your finished top would be 13" across. Do the same for the length.
Is this as clear as mud????