Ayla the trick to this question, as Jan in CA said, is that there are so many unknowns it is hard to give an answer. “Average” adult heads are somewhere in the 20 to 24 inch range, but may be larger or smaller. Given 10 knitter casting on 100 st with 4.5 mm needles could result in hats that would barely fit the 20 inch head or would fall down over the ears of the person with a 24 inch head. One knitter using dk weight yarn could have a hat that is the same size as another knitter using worsted weight. So this is a hard question to answer with the information given.
If you are knitting a watch cap for cold weather wear, the thicker worsted weight would probably be best. However for the watch caps that young people are wearing year round here in the states a lighter dk may be best. A watch cap style hat is easy to start. Knit a swatch with the yarn and needle you like or the person that the hat is for might like. If you are doing a 2x2 rib hat knit the swatch in 2x2. Multiply the stitches per inch by 20 (or the estimated head size). Round up to a multiple of 4 (2 knit + 2 purl) Cast-on and knit. If you use one of the hat calculators Jan in CA provided it will help with the when and where to decrease.
You probably have heard that MMV (mileage may vary) since knitting is a hand craft very few knitter will get the same result even when using the same pattern, yarn, needles and gauge.
We are all here to help each other, let us know more about your hat and we may be able to direct you to a pattern that you might like.
AND please post pictures!
