Uhm, the general reputation of voice surgeries is very bad, and my very limited knowledge makes me estimate the satisfaction rate to 10% maximum. Think about this a LOT and ask MANY professionals before even considering it seriously. This is a serious risk, as it can worsen or even eliminate your voice completely.
As for your question, as with every single surgery (I think), I am sure that young age and healthy weight decrease risks to some extent. But how much is "some" is unknown to me. I do not know WHAT risks it reduces either (voice-related risks, or life-threatening risks only, etc.) It may be very small. As for body size, I have no idea. In any case, I do not think even the best possible parameters could make the operation worth it... Unless there have been some serious advances.