Currently my Cannondale has narrow 38 cm bars. My commuter has 42 cm bars and it is a very big difference. ON my Cannondale my wrists are bent when I ride on the hoods. It is uncomfortable to keep a nice, relaxed straight line with them. My commuter ones feel big still but they really open up my chest and I can keep a straight line with my arms.

How do you know what size to use?

I have narrow shoulders but am top heavy, maybe that is why my Cannondale is hard to hold straight. My boobs get in the way? I find on it I mostly ride on the top of the bar instead of on the hoods and in control of the shifters/brakes.