I'm a beginning surfer, and I want a longboard.
How do I select one? How much should I pay?
What is good width and length, etc.?
If you have never surfed before, avoid going for a really long one - they are very unweildy, can be difficult to get up to speed, and are a pain to carry back to the car when you are tired out :-)
As far as width goes - can you sit up comfortably on it? And can you fit it under your arm?
Islesurfboards in California have this chart, which is a pretty good starting point:
Their website also gives a lot of info on all aspects of surfing.
Additionally to longboards, you could investigate a mini-Malibu ("mini-mal") / mini-longboard shape, with lengths in the region of 7'x" - 8'x".
As a beginner you'll find such a board easier to control than a longboard, and thus less frustrating to use, although longboarders will out-pace you when paddling onto a wave.