Rox,
I used and 8.5" block and 3" inch square.
I found this website which gave me the calculations and then I adjusted a sample till I got what I wanted.
http://www.straw.com/quilting/articles/snowball1.html
For the quilting I plan on outlining the baseballs and then quilting the blue squares diagonally. Since the back is fleece I will not need a batting and will not need to quilt vary closely. I am not worried about the quilting for the backing. This quilt will be going around with us and I just want it to look nice on the back to. I asked in another thread about how everyone did their backings and it seems a lot of people like the idea of a pretty backing. Especially, if it might get seen. I have two quilts which I used a single piece of muslin. It is wonderfully soft, just not to pretty. And as some one stated we work so hard on the fronts why not have a nice backing. This is my first try but I already have another one planed.
G