aka Tom Reingold
Join Date: Jan 2009
Location: West Village, New York City
Bikes: too many
The answers to your question, in order are:
1. I don't know, but I think you have to make educated guesses.
2. Yes, usually, but there are fixed stems that are very tall.
3. Measure on two axes.
It actually is possible to do this scientifically. You could find a shop that does FIT sessions. I think they're about $150, and they spend an hour or more with you. They put your bike on a trainer and observe. And they put you on a special stationary bike whose every dimension is adjustable. But it's probably premature to do that, since you haven't ridden your current bike much yet.
And remember, your tastes and needs change over time, further complicating matters.
I recently got lucky. I bought a bike to add to my embarrassingly large collection. It wasn't a big purchase or decision, since it was underpriced on craigslist. It felt odd at first, and after a while, I realized this bike fits me better than any other bike I own. I plan to duplicate the dimensions on my other bikes. On this one bike, I could sit on it all day, practically living on it.