Well, you can use a larger hook, an I or J. Look at the numbers for the different sizes; usually there's an even amount of stitch increments between them, something like 8 or 10. Just add to that, going up to the sizes you need. It's better to look at the actual measurements, rather than to figure you need to go 5 sizes up. A size 12 may not correspond to a dress or sweater size 12, but might actually be larger or smaller.
sue- knitting heretic