As skier232 said, skis come in different lengths to suit different purposes and different bodies.
First, getting good-fitting boots is vastly more important then getting skis. If your boots don't fit well, then you can't possibly control your skis efficiently, at which point you might as well be sliding around on a pair of cafeteria trays.
As for skis... As well as varying lengths, they come in a mind-numbing array of shapes as well. Narrow skis that are good for racing, wide skis that are good for deep snow, in-between shapes that work well anywhere, skis that are good for doing tricks in the terrain park, etc. It's worth heading down to your local ski shop and talking to the sales people about what's available.
But GET BOOTS FIRST. If you post your location here, somebody can probably recommend a shop that takes the time to fit your boots properly.