I got my fabrics from an I Spy swap on this board a couple of years ago. I think they were about 5"...yours don't need to be larger than 4 and a half inches. You can use sashing or a pattern like Attic Windows to enlarge the size of your quilt if you think it's necessary.
I think most of the fabrics in our swap came from Joann's...I recognized them from the store
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Good luck. I'm sure your quilt will make some special little someone very happy.