If you're looking to make a quilt you can always visit www.quilterscache.com, they have lots of free patterns, then when you find one you like you decide how many blocks it will take to make the size you want, and then find one of those free "fabric calculators" (seems a good portion of the online stores have them or you can just google fabric calculator) and that will tell you how much fabric you need to buy for the size quilt you want.
Other options for patterns are the fabric manufactures (you mentioned Moda so here's their website's free patterns http://www.unitednotions.com/un_main.nsf/free_patterns) they always seem to have beautiful quilts, and they will tell how much fabric to buy to make the quilt, and of course you can always change the pattern to suit your colors and or fabrics, no one said you HAD to use theirs, those kind normally give a finished size and if it's not the size you'd like try to keep in mind that you want to "leave off the border" or "add two more" ect.
When I'm stash building I buy 1/2 a yard off the bolts that gives me two fat quarters, one to keep and one to share. Though so far all the fabric I bought that way was "shared" so perhaps a yard would have been better... Truly to buy a half a yard is to pay have the price listed on that bolt, so it's not so expensive, and you get to take it home and cut it into two cute little fat quarters (or leave them whole if you don't want to share)
When you find a fabric you LOVE, and you will know, buy as much as you can, you never know if you'll see it again or what you might use it to make. For instance for a quilt back you need something like this:
TWIN - 63" x 87" = 5 1/4 yards
And that would use up a lovely "favorite" fabric in no time. :cry: :cry: