Female boas come in different sizes and so do the handler. Impossible to give a definitive answer.
Myself I have never had an issue holding a boa by myself in about any manner I choose. The rules I have for myself if it isn't my snake I will not drape it around my shoulders even if the owner or pet shop says it's "puppy dog" tame. My snakes I have no issue since I have a better handle on their moods. The other rule I have is I will not allow any snake (including mine) regardless of size to allow it's head get close to my face. I've seen the result of a snake bite to the eye area from an "expert handler" customer at a pet store years ago. I think his passion changed to fish that day.