I have a 52 on my Shiv, which has 165mm cranks, but I have a 54 on my P2 that has 170mm cranks. I wish I could say I landed on that after hours of testing but the honest truth is that's just the way those bikes came. I have not, as of yet, played with changing chainrings. I find it much easier and economical to test cassettes.

I will say this, however, when I decided to try my hand at St. George, knowing that course is notoriously hilly (particularly Snow Canyon Road), the P2 was almost instantly eliminated from consideration. The smaller chainrings on my Shiv gave me better options for climbing big hills. Looking back, I didn't give up anything on the flats or descents with the Shiv but it was a much better choice for the climbs than the P2 would have been.

So, my take is you have a couple variables to figure out. First is how much power you can put down. If you're a monster, averaging a couple hundred watts over the course of a ride, maybe bigger is better for you. If you live and ride exclusively in flat-ish territory, maybe you can make better use of 54 or better.

My thinking is if you're on flat ground and holding a steady high effort pace, look at your cassette. If you're not on one of the three smallest cogs on that cassette, you don't need a bigger chainring.

But that's just me. I'm sure other, better educated opinions will chime in.