I've googled quilt as you go and the best instructions I found were here.
http://www.quilterscache.com/H/howtoquiltoneatatime.html
Some other I looked at
1. Said cut to exact size
2. Said backing and batting should be 1.5 inches wider than the top.
3. Said 3 inches extra material.
Since this is your first project I would make your first block with the 3 inch overhang to see how it works out. If you have alot of material left after quilting you can go alittle smaller on the next block. I always leave room for mistakes. It's not like a haircut, if it's cut too short it won't grow back.