Wolfkat, first of all welcome to the board and the wonderful and sometimes crazy world of quilting. Now, to answer your question. If you are planning to sew these with the usual 1/4 seam allowance then you would need 25 squares across by 25 down. That would make a total of 625 squares. That should get you your 90x90 measurement. This total is just squares. The total would be less if you are planning on adding borders. Hope this helps and good luck. You must be planning on having alot of down time or else you sew really fast. Let us know if you need anymore help. Everyone here is so helpful and friendly. It was not long ago that I myself was new to the board and had tons of questions. The members here really helped and inspired me to spread my quilting wings and take off.