Within a single model of ski, going to a longer length usually does the following:
a) Increases the total weight of the ski - This makes it more laborious to carry them, shuffle around on them, they pull down on your legs more on lifts, etc. If you are into pre-jumping turns, it makes it harder to lift them off the snow, etc. etc.
b) Increases the "swing-weight" (aka polar moment of inertia) of the ski - This means that there is more material that you have to swing around, and this material is further from the pivot point (the center of your foot). In swivel / pivoting moves, (eg, on top of moguls), this makes the skis much more difficult to swing around to the desired direction using twisting action of your feet.
c) Increases the sidecut radius - Most model lines of skis have a specified tip, waist, and tail width with a nearly circular curve connecting these along the edge of the ski. Make the ski longer and the radius of this circle increases even if the sidecut (width) dimensions are unchanged. A larger sidecut radius means that if everything else is equal, the minimum radius of turn you can carve without skidding will be larger.
d) Often (tho not always) increases the longitudinal stiffness of the ski, particularly in the critical region underfoot. This is something that the manufacturers can decide to do or not do. Typically, they do it because larger (heavier, taller) as well as more experienced people buy longer skis. Generally, the stiffer a ski is, the harder it will be to carve into turns, especially in soft snow and/or at low speeds.
e) Increases the ability of the ski to float in soft snow - More surface area equals less pressure (pounds per square inch) compressing the snow. This is obviously useful in powder, however, you can also increase the ski width and get the same effect. With either, you stay more on top of powder and so (all else being equal) you go faster and turns are easier if you have more surface area under you.
f) Increases the "smoothness of the ride" - Long skis tend to average out terrain irregularities better in high speed long radius turns.
g) Tend to vibrate at lower frequencies in racing speeds on rutted terrain. This means that they are in contact with the snow less, hence less control (again, only if all else is equal such as internal vibration damping, etc.)
I'm sure there are lots of other things, but I can't think of any more right now.
Hope this helped.
Tom / PM