I started like how every kid started back in the day. Which was bone stock 14" wide on most all Razor Models (A,B). Now when RAD came out with Solid Bars, I got 15" wide and 19" tall. It was great. Just a little wider. Then when I got Buff Bars for the first time I went up to 16" and I road that in aussy at the x games.
From there I went up to 18" and hated the jump so I cut them down to 17" and road that for about another year. As soon as I was done with 17" wide I then jumped up to 25.5" wide with back sweep, up sweep and a 1/2" rise. Kinda redic. (down hill bars)
This all of course is width, Not hight. When I had to ride stock bars back in the day, there were only adjustable bars like how they are today so I never had a set bar hight. But right between the knees and hips is good for your center of gravity. For me by bars were around 19" to 20" tall.
I ride 21" wide and 20" tall right now. But that changes every year or so. I'm 5'4 by the way.