I didn't cut the borders first for the same reason, I think - I didn't know how much to cut. This is what I ended up with from the first one I did:
(1) the 5" blocks were laid out 7 blocks wide by 8 blocks down (5 colored blocks by 6 colored blocks, surrounded by the light blue blocks that make the partial first "border"
(2) the second light blue border (not involved with the twister cutting) ended up being cut 18 1/4 by 21 1/4.
Don't know if there is a ratio there to calculate other sizes. Here's a photo of the unironed finish: