You should be able to determine whether a hat will fit your head by doing a little math. First, multiply your head circumference in inches by 90% (for a snug hat) or 95% (for a less snug hat). This will tell you your ideal hat circumference. (You want it to be a little smaller than your head so it stays on.) Multiply this number from the gauge (in stitches per inch) that you get from your swatch. Assuming you're doing a bottom-up pattern, this will tell you how many stitches you need to cast on for a hat that fits you. If your pattern calls for significantly fewer stitches, you will know that it will be too small. You can compensate by casting on more stitches (do it in a multiple that will work with your pattern), or trying a larger needle size (or even a different, thicker yarn) to adjust your gauge.
Hm, I'm not sure that I explained that very well. Really, the easiest thing would be to use a hat recipe like the one Ingrid linked.