Translation:I want a girl with a short skirt and a long jacket.
Hehe, I never thought I'd see the day!
Yes, you would use "et kort" when describing a neuter noun such as "skjørt", and "en kort" when describing either a masculine noun or a feminine noun for which you're using the masculine declension.
et kort tre (n.)
en kort trapp (m.)
en kort linje (f., masculine declension)
ei kort linje (f., feminine declension)
As you can see, the adjective "kort" uses the same form for all three genders of nouns, and it's only the indefinite article (en/ei/et) that changes.