I assume you are talking about the type of bars that come on mountain bikes? Are these truly flat bars or are these riser bars? Flat bars typically are 23" wide but they can be found wider. Riser bars will come much, much wider for downhill mtbikers. They all can be cut down.

I used to use flat bars 22" wide because that was the standard years ago. I discovered I liked a wider bar because I wanted the slower steering of a wide bar. My XC mtbike has a flat bar 23.5" with bar ends. My freeride bike has no bar ends and a riser bar 25" wide.

To give you a size comparison, I am 5'3" with somewhat broad shoulders.

I am putting a new flat bar (Salsa Pro Moto) on my cyclocross bike which is set up as my townie. I'll cut it down to the same 23.5" width as my XC mtbike.

This best advice I can give you is to consider the breadth of your shoulders and the steering (narrower makes it faster; wider makes really tight turns and narrow spaces harder).