I'm 5'1 and I do karate and boxing. Personally in karate I find it hard because generally, people who are taller with longer legs get the points for head kicks (even if I can reach that high I have to be considerably closer)
I do know someone who is 4ft something and deliberately has their stance really low and uses this to their advantage, as the taller person will have to strike downwards and will find it hard to hit with body shots, so will be limited to headshots.
Boxing however... If you're fighting a taller person, if you keep your head down and your guard up, the taller person will have a harder time getting underneath your guard than you will of uppercutting them from below :P
So I'd say in a street fight, a strike upwards to the sternum is defo gonna slow them down, you'll be able to wind them.