Therealclint:
Don't confuse the suspension capabilities of the bike with the engine size, they are two totally different things.
In general, dirt bikes are sold in two major catagories: MX (racing) and "off road".
The MX bikes will have better suspension and high performance engines. The long suspension travel often means a tall bike (which wouldn't be a problem for you). The high performance engines can be harder to start, a bit more "finicky" and not as beginner friendly. They also cost a lot more.
The "off road" bikes are mostly all "detuned" four strokes. The lower compression ratios make starting easier and allow the bikes to run on regular gas. The engines are more forgiving to being "lugged" and will be much more beginner friendly. The seat heights are often lower to accomodate smaller riders.
If you expect to be hitting jumps or riding agressively through the rough stuff then you should consider a MX bike for the suspension. If you are just going to ride the trails at a more leisurally pace then the off road models might suit you a lot better, especially as a beginner.
As for the size of the engine: A 125 two stroke or 230 four stroke will have plenty of power, especially for a beginner. I would not recommend starting off on a big bore bike.
Rod