A short is just that... short
A short is just that... short
A short screen play has no real limit on length...
but once you've broached the 90 to 120 page situation, then you're definetly out of the short film category.
I've seen short films range anywhere from 30 seconds to 45 minutes.
Once you go beyond that, it get's closer to feature length.
For film festivals, it's good to stay around 3 to 5 minutes, as they are easier for film festival planners to program into the festival.
However, do not restrict yourself.
Write it first.
Then do a second draft.
Then a third if necessary.
Finally polish up your script...
and then see how long it is.
Who knows, maybe you'll write a brilliant short, or a wonderful feature.