Originally Posted by
momocycler
Jonathan, you mention "Going to a larger size frame will tend to stretch you out more, and it will bring the bars higher relative to the seat."
I should add some caveat about the above being true for a given frame design. A larger Rockhopper will stretch you out more than a smaller Rockhopper. But if you change frame designs too, then all bets are off. I've an 18" Fargo frame with a shorter top tube than my 17" Rockhopper frame, for example.
I should also add that sizing trends change over the years. I see a lot of older mountain bikes with long stems and shorter top tubes than what is common today. You can't compare an 18" frame from 15 years ago with an 18" frame from today. They may or may not fit the same.
I tend to agree w/njkayaker. It probably is worth your time to visit a couple of bike shops and seek some advice from people who can see you on the bike.