"It is really a short skirt."
Translation:C'est vraiment une jupe courte.
"It is really a short skirt" and "It is a really short skirt" are different in English. The later uses really as a modifier of short whereas the former would comment more on the general nature of the skirt being short as opposed to perhaps medium length. Does the word order alternative noted by Sitesurf have the same implications in meaning?
c'est tellement une jupe courte is not correct, because you would miss a clause afterwards: c'est tellement une jupe courte que ...
c'est une jupe tellement courte = this is such a short skirt
c'est réellement une jupe courte = this really is a short skirt
c'est une jupe réellement courte = this a really short skirt.
You never use il/elle est in front of un(e). Here is a breakdown of when to use il/elle est vs. c'est: http://french.about.com/library/weekly/aa032500.htm