too small?
52 or even higher?
For anyone whose Z2 is around 180-220W, i'd say 53T at least, and preferably 55T. That way you spend more time in the middle of the cassette. Mechanically it's better to have a big ring and be in the middle of the k7.
50T on a TT bike, unless you're 50kg, means you're spending most of your time in the smallest cogs, which you shouldn't. And it looks noob AF.
Also, now RDs all handle 32T or more as largest cog, so while it's nice to run a 12-25 because the jumps are small, i think it makes more sense to do 55T with a 11-34, or 11-36.
Obviously that's assuming modern equipment with fast tyres, a half decent position and all that.
Also from experience, in TT position, a lower cadence feels nicer and more sustainable. I think it's tested faster too. If you're comfortable doing 95 cadence in TT, good for you, but i'm not. A TT is usually about motoring at a sustainable pace, so you want the motoring speed to be in the middle of the cassette, at the most natural cadence for you.
Also, you get to flex your big ring and impress... nobody :p