What amperage (size) battery should I use for a 150hp Yamaha outboard engine?

Question

outboard engine is aprox. 10 years old

Answer 1

Assuming you know the specification, you want a battery with at least the CCR specified, but better yet, the largest CCR and reserve capacity that will fit in the battery case, usually a group 24 or 27.  Also the longer the warranty, generally the better the battery.