Bicycle Tutor

I'm considering an adjustable stem on my 56cm road bike because I want low and aggressive to go fast at times but I want to change sometimes for a more upright and casual ride with the grandkids. I was wondering about the length of an adjustable stem. I've seen 80 mm and 150 mm lengths and everything in between. I'm unsure what to get.
Adjusting the stem upward would in effect shorten my reach for an upright ride, right?. Conversely lowering the stem would make my reach longer giving me a more aggressive posture. I'm thinking that because I have short legs and arms compared to a long torso I might be better off with a shorter stem length where as a longer one might possibly be too much for my short arms. Is my understanding of the principle correct?

Proper bike fit is fairly complex and involves more elements than just stem length. Short legs/long torso might actually point to a long stem. If you search on the web some there are resources for doing accurate measurements of your body for proper fitting. If you're doing a lot of miles, fit becomes more important to avoid injury and get the best performance. For the casual rider, "does it feel comfortable" is a more relevant question.
But I'd start with what you have now. How long is your current stem? Does it feel comfortable/too short/too long? I'd think the adjustable stem in the down position will basically mimic what you have now - so decide based on that.
Adjustable stems will both raise your hands and bring them back toward your body. I woudl think the change is probably slightly greater in the up direction rather than the back direction since you're starting at flat (0 degrees) but probably won't go all the way to vertical (90 degrees). A longer stem will change your position more than a short one.
One alternative - there are secondary brake levers you can put on the flats of your bars (similar but much better than the "safety levers" from your old schwinn). They allow you to ride up on the flats of the bar but still have brakes directly under your hands. Also called "Interrupter Levers"
http://www.sheldonbrown.com/harris/brake...http://www.sheldonbrown.com/harris/brake-levers-drop.