The biggest you can get, psych out the competition.
i had the same dilemma a couple of years ago, it will ultimately come down to a few things, terrain & distance likely the main ones.
you can try and use a gear calculator type tool to try and get a gauge based on your cadence, realistic speed & then the aim is to have a straight a chain line as possible so where that will land on your cassette but in truth you’ll unlikely find a one size fits all solution unless you only ride 25m/40km TT’s.
i have 54/56/58 and they all see use throughout the season
there is a bit of discussion around 1x in the time trialling thread