The formula I use most times when making a quilt top without a pattern is ; I make a bunch of squares put them up on the wall, it it's too small I make more, it I have too many I use them on the backing, or make another quilt top.
Really, There is a 'standard' size to a twin, double, etc.. if you search for mattress size, you'll come up with it, then decide how far over the edge you want your quilt to hang, that's your top size. Divide that by a block size, say 6 inch, and that's how many blocks you need. Easy, don't forget to show us a pic when it's done!