Most fabrics speak to me or I already know what sorts of fabrics I'll use in piecing or in backing etc. If I find something I really love but can't figure out what I'd do with it in piecing, I buy enough to back a large quilt. Fabrics that I know I will use in piecing without a problem, I buy anywhere from a fat quarter to a yard or three. Large pieces get used for sashings, bindings or larger plain blocks and the smaller pieces I use in piecing HST's and other small block sections. The longer you quilt the better you will get at judging what you'll really use and what you won't. For example, I love looking at novelty prints but know that I will seldom use them in a quilt...they are just not part of my quilting style. However I would use them for a backing for a particular person that they might remind me of so in that case I would buy a large length of it just so I will have it the next time I get ready to make ______ a quilt.