Using only someone's height for comparison will still leave you guessing. That person may or may not have the same inseam and trochanter measurements you do.

Giant's sizing on the Trinity was been wonky for a long time. Now that they're showing stack, reach, armpad stack and armpad reach, you have a better chance. The Trinity is not a bike I would buy without a proper fit, or at least the recommendation of a fitter who has experience with the Trinity.

I am 183cm tall and just ordered one a few weeks ago. It won't arrive until late Feb or early March so I haven't actually ridden it. I used the pad x/y from my current bike and it is comfortably in the range of adjustment for the medium size. Assuming your current bike fits you I would take measurements off of it.

I am 177cm tall and ride a small Giant Trinity.