"Isn't it short?"
"Is it not short?" would probably be the closest. Not to confuse you, but it depends on the intonation. If a simple question it would mean 'Is it not short'?, but if asked in an incredulous tone it would mean 'It's short, isn't it?". The same could be said for the same phrase in English, depending on whether you emphasize the 'it's short' or the 'isn't it'.
"It's short, right?" would be 「みじかいでしょうか？」or similar variants.
The negative form of 短しい is 短くない.
い-adjectives form complete predicates, thus they never take plain copulas like だ; です is tacked on for politeness only.