Prism99

I would just sew together 2 rows at a time -- 2 sets of one block, 2 sets of three blocks, 2 of five blocks, etc. until there are just enough blocks left for the middle row.

Edit: I think graph paper might be better. Get out some graph paper, count each square as 4" finished (assuming you have 4.5" unfinished squares), and start coloring squares in a diagonal line. From this you can figure out the finished length and width and also how many squares are needed.