if you're quilt measure 36 inches and you want to figure out what sizes you could use - i would go with any number that divides into 36 equally.
36/2 inches = 18 blocks
36/3 = 12
36/4 = 9
36/5 = no good, its not a whole number
36/6 = 6 blocks
36/8 = no good, its not a whole number
so, you get the idea. now you should decide what size block will look good with the style of the quilt. that really just comes down to what you like and the look your going for.
the cuts you have to make to achieve the correct finished size is 7/8 larger for triangles and 1/2 larger for straight cuts - this accounts for the seam allowances.