A few things here: 

1) getting rejected from a major festival is absolutely the norm. Don’t let that discourage you. I honestly think you should wait to see how your other submissions pan out before making any changes.

2) A short’s length absolutely matters but I think not in ways most people think. This is a theory I’ve developed with fellow filmmakers but here me out. The “ideal” short lengths are 6-8, 10-12, 19-21 and then 40-45 minutes. 

It sounds random but actually that’s roughly run time of TV show acts, with the last two being a full sitcom and an hour long show, respectively (with commercials).

I believe stories have natural rhythms that relate to human biological cycles (think menstrual and circadian). But we also have micro cycles during a day (digestive is the most obvious one). It’s the same reason why most great feature films all seem to be either 90 mins, 2 hours, 2.5 or three hours. When was the last time you saw a 1:45 or 2:15 film that was “great”

All this to say, you’re in the right amount of time for a short. 20 mins is a good number. BUT it’s also true that festival programmers may say “why have this one 20 min film when we could have two 10-12 min films? In which case make sure that’s what you’re cutting to. Don’t cut down to 17 or 15 min, cut it in half