I found that for intermediate skiers length of the ski should be about equal to your height.  Length makes a difference, longer means faster, shorter means less speed more maneuverability.  If you want to carve, shorter ski will give you a better turning radius, however going too short will take the fun out of it. 

Shape of the skis is definetly helpful, nose and tail should be wide, and waist should be narrow.  From my experience, increase in waist width will significantly slow you down, so make sure your skis have a lot of shape to them (they will look like a peanut), where the nose is usually the widder part, tail is slightly narrower, and waist is very narrow.  Shapped skis help with control and initiation of turns, they are one of the greatest advancements in the sport industry.  They're worth the extra dollar if your thinking about upgrading.