Wire Size
Your breaker sizes are a function of the wire sizes. What size wire do you have from your alternator B, and Alternator Field, and what size wire goes to your starter relay?
In my case I have 16AWG to my starter relay, and this line also feeds auxiliary power, so I have a 10A breaker. (Up to 15 is allowable)
My alternator is a 60 amp, with 6AWG so it gets a 60A breaker. (Up to 80A)
My alt field is 16AWG and has a 5A breaker. It's okay to have a smaller breaker for the wire, but not the other way around.
See AC43.13-1B para 11-49 and table 11-3.
Don