Right, I know, you're reading the title thinking that old fart Pcad wants to know your racing age.
Your big chainring tooth count. I have ordered a 55. In training for TT's it seems to me the difference between a 55 and a 53 is only on click of the rear derailleur, but more speed on the descents, which did strike me as a key factor in going fast on rolling TT courses. I considered a 54T, but that seemed rather pointless, so I opted for the Big Mama 55.