How do you determine what brim size you should select?
Well this is a difficult question for me to answer because lot depends on your personal taste
and your community that you live in.

For example if you are in Texas you could easily get away with wearing a 5" brim
and you would fit right in.

but if you were in Down Town New York City then a standard size brim would fit in much better,
based on the community.

But also factored in this is your personal taste
As well as and your personal body build and character.
All of which i am not able to answer for you,

but generally the larger the person
is the larger the brim can be and still look in proportion.

a taller skinner person might want a shorter brim say someone that is 6" tall 140 pounds.
but a person that is 6 foot tall and 220 pounds would generally like a little larger brim.

How large or how small you would have to take into consideration the other factors
mentioned earlier to determine that.
So sorry i couldn't be of much more help on that

but also keep in mind that wider brims tend to look more western
Shorter brims look more Fedora style and more city style.

The brim can change the look quite a bit so give it some consideration
and if you are thinking of a large brim error on the conservative side
a little short rather then a little larger.