I made one with I 'think' 4" bottom square and 5" top square and it covers a standard size bed with just a little bit of over hang. It's WAAAAAYYYYYY to big for my washer and waaaayyyyy to hot to sleep under. Sure is pretty tho. lol Beware - it's real easy to OVER STUFF the squares and it makes them really hard to sew around. Less is best.
Edited to add: I couldn't say how much material it took - I had just cut up a bunch of squares and started sewing them together.