There are lots of good thoughts above; I have also pondered the weight question quite a bit as I'm 5'8" and 120lbs.
There are pros and cons to being a small, lightweight. If you are racing on a weight-limited crew, your lack of mass at the helm can allow you to have a beefier mainsheet trimmer or extra rail meat. On the other hand, if you are racing something that requires the skipper to also be ballast, it can work against you.
Here's an interesting article (Canadian Yachting Association - National Sailing Teams
) Note the sections on the 470 vs. the Star. You'll see that the average weight for the Star's crew is 420lbs - you're going to have a tough time finding a 320lbs, lean, mean crewing machine to help you out in that campaign.
From a leadership perspective, humans are drawn to physically dominant leaders. The implications of that are vast and should be considered from multiple perspectives. In the end, the skipper needs to be the source of strength on the team - everyone looks to you when situations aren't ideal.
From a physical perspective, you'll have to consider your job on the boat and the equipment available. A 100lbs person is at a disadvantage if there isn't sufficient mechanical advantage on the mainsheet or if you need to take over some other high-power job.
I'm 5'0, 100llbs, will I make a good offshore/dinghy sailor? Also, because of my height/weight, is it possible that this could affect me negatively if I want to skipper?
To answer your questions directly;
Q1. Will you make a good offshore or dinghy sailor?
A1. Your sailing aptitude is not proportional to your height or your weight.
Q2. Is it possible that this could affect you negatively?
A2. Absolutely. All else equal, a boat owner will not hire a skinny, mediocre skipper if there is a strong, strapping, mediocre skipper available. The same reason they'd elect him class president, pick him first to play baseball and be picked up at the local bar. You've got to work harder than the next guy... or buy your own boat. :-)
... I have quite a few thoughts on this subject "as crew" but will save them until the question is asked.