I have a MTB that I converted to a single speed. Because the dropbar I wanted to use would not fit trough the stem I ended up cutting it down quite a bit. The result was a 12 INCHES long handlebar. I found it surprisingly stable to ride.
I figure a handlebar about the width of my shoulders is a good all round length.
The 12" long handlebar on the single speed MTB.
The bike has cantilever brake calipers on the front and an under the chainstays rollercam brake on the rear.
Cheers