It can be tricky to style a skirt that goes a little below the knee if you are not tall. For me (I'm 5'2"), I find that heels help a lot.
You may also find that footwear that is low contrast to your skin (or tights, if you're wearing tights) will help lengthen the leg. Pointy-toed footwear is helpful in this regard too.
I have also found that wearing knee-high boots can work, even if they are flat. Since the top of the boot will be hidden by the skirt, you have a long line of leg plus foot that has a lengthening effect.