IMHO 7ft will be too short for a first board. I'd look at something around 8ft, but nothing under 7'6". Once you progress, you might get into shorter boards, but by that time, you will have gone beyond a second hand starter anyway.
The more important factor is probably thickness and width rather than length though, because the wider the board the more stable and easier to learn (generally) and the thicker, the better it will float you and the easier it is to paddle and catch the waves in the first place.
The other thing to consider is whether to buy a hard or soft board. If you are committed and plan to get out every day and are prepared to take a few knocks, then get a fibreglass board. If it's only going to be a once in a while kind of thing - like on weekends - to start with, then maybe a soft board might be a better way to start, for this summer at least. They also have the advantage that you can give it to your girlfriend / little brother / mates and not worry about it getting damaged and they will have a lot of fun.
I'm a little biased, I'll admit because my best mate owns the company, but if you are leaning towards a soft board, take a look at G-Boards (www.gboards.com.au). I know they have US distributors and the boys there will be able to put you onto the right person.