I have always used 140mm rotor for my triathlon-bikes as they have enough braking power and are lighter than 160mm, and maybe more aerodynamic.
I see that 160mm looks like the standard choice. Is there any good reason for it?
Depends where you live & ride.
I have bikes with 140 & 160.
My rides are relatively flat with few steep & long descents.
Im happy with my 140s. Feel they are powerful enough, to think I was on rim brakes before and already happy.
If I could convert my frame that has 160 specific mounts to 140, I would.
Again depends where you ride.
I’ve seen a number of bike brands doing 160mm in the front (for the better braking) and 140mm in the rear (for being lighter and aero). I really like that compromise, and so I’ve set up all my disk brake bikes that way, so I can use the same wheels from my TT bike all the way down to my gravel bike.
The biggest benefit of a 160mm disc is the heat dissipation. Helps with long descents and especially long, technical ones. I do feel this is much more applicable to road bikes than it is to tri bikes though…
My new bike came with 160s and a good set of wheels. I converted my older roadie with 140s so I can swap the wheels around. I did actually notice better brake feel but other than compatibility I’d be fine if both bikes were 140.