It's the thickness that's important to the strength.
Bottom line is make it thick enough not to break and the rest is down to comfort.
If it's twice as thick as the limb it will be 8 times stronger, so then if it's half the limb width that would still be 4 times stronger.
If it's 3 times as thick as the limb it will be 27 times stronger! So you could make it really narrow (1/8 of the limb width!) and still get away with it.
Of course in reality it's all about blending in the curves to avoid sudden changes of section and stress concentrations, also if you went too narrow you'd lose rigidity from side to side.
Hmm, I expect that's as clear as mud.
Del