More than length, I noticed that Spanish women tend to wear their skirts (all clothes, for that matter) with a bit less "ease" (the designing term, meaning the amount of extra fabric in the garment beyond your own body measurement--if your bust is a 36, you might add 2 inches of ease for a close, but not tight, fit) than American women. A bit more form-fitting, a bit more stretchy/clingy than ours.
That's not to say that they are all in skin-tight clothes...just that theirs seem to reveal a bit more about the figure beneath than we are used to.
So if you're going for a long skirt, you might find one with a tighter fit in the hips and a narrower width at the hem...and for short skirts, a bit tighter (and shorter, if you have the nerve!).
But really, I hate to rain on everyone's parade here...the sad fact for those of us trying to blend in is that Spaniards (like Americans or anyone for that matter) have excellent radar for spotting foreigners. Walk into a shop, and you can be pretty sure that you're going to be pegged as a foreigner before you open your mouth to speak! I remember hanging out with a group of (all Spanish) friends, wearing clothes and makeup purchased in Spain, and having a newcomer to the group ask "quien es la americana?" before I said word one! Blending in is more than looking the part--but it's still fun to try! I tend to think that the most fun way is to shop there, at least a bit. I always try to wear a new Spanish outfit home on the plane...and I HAVE had American flight attendants take me for Spanish, but Spanish ones would probably still know I was an imposter!