I'm currently riding an 100mm stem, but I feel a little stretched out. I'm not uncomfortable to the point of pain, but it does feel slightly strange, and I do feel like my arms should be more bent (a.k.a. not totally straight). Would a shorter stem change this? Does it sound like it'd help me?
In all honesty, it is hard to say since bike fit is dependent on other factors as well, i.e. top tube length and saddle fore/aft position. Why don't you visit a shop that you trust and ask them? Alternatively, if you have a local cycling club, you could get their input. Sorry for not giving you a "yes or no" answer and not to beat a dead horse but fit is something that has to be seen, not just described in words.
if your arms are stretched and straight out, then yeah, you have a bit of a bike fit issue. a shorter stem would probably help, but like metabike said, it might be you have a large top tube, the frame is too large for you, the saddle might be too far back, etc. you can adjust your saddle, you can't do anything about the bike frame, and you can adjust your stem lenght/height. go to a bike shop you trust and they should help.
1984 Bridgestone 400 1985Univega nouevo sport 650b conversion 1993b'stone RBT 1985 Schwinn Tempo
There are a number of variables which have to be considered. First is your own anatomy, ie. your arm length and upper body size; others are top tube length, saddle position, and stem height. One size doesn't fit all. One place to read about handlebar positionin is on the Rivendell site, www.rivbike.com. Grant Peterson of Rivendell opined that 95% of all bicycle handlebars are too low. This is because that Bicycle manufacturers think that everyone wants to be a racer.
I ride bikes with threaded headsets and quill stems and have replaced the original stems with Nitto technomics. This has made all my bikes much more comfortable. Unfortunately because of the yen's rise in value in relation to the dollar, Nitto products are becoming quite pricey. If you have a threadless headset, there are stem extenders, I believe.