That's a lot of yarn and squares. Cool! You could also do them kinda like a crazy quilt (all different sizes and shapes of fabric sewn together: http://pinterest.com/pin/117586240242117559/
Do you have a place you can lay out your squares and see what they look like "fitted" together? That might give you an idea of what sizes and shapes you still need (to fill gaps), and it will also tell you what the final size of your creation will be--and what "shape" you can make, too (rectangle? square?).