IMHO it is not the width of the beam, but the width of the transom that is the critical dimension. I would be seriously concerned in bad weather if the bow/stern of the tender is sticking out significantly and I was beating to windward. The other critical dimensions are buoancy in the stern and height above waterline - significant when running before the wind in high seas.
-------------------- "Be wary of strong drink. It can make you shoot at tax collectors - and miss."
Robert A Heinlein