Take a picture of your crank and B/B connection, I can usually tell by looking.
or
If you have the brand/model of the existing crank, that would help determine the B/B, too.
About leverage & crankarm length..
As a reference, I used to use 175mm crankarms when I was racing 20+ years ago, trying to be fast on the bike, not riding long distance or pleasure.
Nowadays, I ride for pleasure.. for company.. for entertainment at destination, not to exert myself to the limits, I ride 170 or 165mm cranks and spin at 85-90 rpm cadence most of the time.
I find myself riding further, getting off the bike with less pain with shorter crankarms.