What's the gauge - how many stitches per inch? 100 sts divided by sts per inch gives you how many inches your cast on will be.
There are complexities to adjusting size - it depends on the type of pattern (close fitting or loose, shaping?), the type of yarn (cotton), and the type of stitch pattern (plain stockinette?)
If you have your measurements, you could be brave and try to adjust the pattern by taking your gauge and multiplying it by your measurement to see how many sts around you need, but that's really simplistic.
Here's one link online: http://knitty.com/ISSUEfall02/FEATdiyknitter.html