To the OP; one suggestion is to save your money until you can afford the bike you really want, like the Sub 10. In the meantime, used rigid (no suspension) MTB's make great commuters with the addition of fenders, lights, slicks, and a rack. By doing this you could likely get started for less than $200, and then next year buy your dream bike. That would also give you an extra year of research and test rides. BTW this is pretty much what I did and now I do have my dream bike.

You could start with something like this: http://sfbay.craigslist.org/sfc/bik/1869223385.html

Of course that's just a suggestion; the Sub 30 is a nice bike if you can find a way to swing the finances. Just remember to budget at least $100 for accessories!

To COGGIOLA: The frame size is usually written on the seat tube (the part of the frame in which the seat is placed). Look down near the bottom bracket (where the pedals are). Failing that, just ask a salesperson at the LBS. The P4 looks to me like it would be able to handle a person your size without a problem.