Yes, size DOES matter! And material type, and thread type (fine or coarse) - AND is it dry torque or wet torque? Dry torque is what it sounds like; wet torque refers to anti-sieze or oil applied to the threads AND under the head of the bolt or nut. Due to the reduced friction, a bolt torqued wet is under much more strain than the same bolt torqued dry.
If you can, I'd contact the manufacturer of your boat and get their recommendations.