Well board length is determined by height, preference, and skill level. So you're light, which is part of it. You're also a new snowboarder, so you'll want one that's easier to control. If you prefer a longer ski, then perhaps you could go a bit longer on a snowboard as well.
For people our size, going "short" means dropping below 140cm. I feel most comfortable on boards in the 143-145 range. Longer boards are more stable at high speeds and better in powder, but also harder to control. Shorter boards are better for park, ground tricks, etc. For you, a 145 is a good size for all-mountain riding.
The Infinity-R is a great board, but probably not the best to learn on. Look elsewhere (perhaps the Pandora-R) if you want something that's forgiving.
The Infinity is great on powder, even at shorter lengths because of its RC technology, which helps keep it afloat.
I have the 2010 Infinity-R. The 2009 was produced both with and without the rocker technology. It is now only produced in the rockered version. I ride all-mountain. No park, but I really enjoy buttering and other ground tricks. I love the powder and the trees, and I spend a lot of time cruising the groomers as well.
Hope this helps!
Last edited by lilfoot1598; 11-20-2009 at 02:17 PM.