As others have alluded to, the rim and tire width pairing is more important than the absolute width of the tire. First, consider the wheels that you may own, since that would dictate things. If you have flexibility there or have yet to make a purchase in wheels, consider the courses that you will time trial on and select something that will pair appropriately, for example go wide if the courses warrant 28c. Just make sure to pay attention to outer rim width and tire width and as always, make your own measurements since different combinations have different real world measurements.

Lastly, there is a wealth of information in Slowtwitch and some of the British time trialing forums about this stuff, highly recommend. Our own "33' racing subforum used to have a pretty trafficked TT thread, but things have been a bit slow. Good luck

Lastly, as CAT7RDR mentioned, if you have flexibility in equipment, empirical testing works well and you'll find that each athlete has their own unique optimization. The Chung method is fairy easy to execute with the free version of Golden Cheetah and the aero lab extension. The method isn't too hard to learn.