First of all, that's awesome that you've got this ambition to compete in triathlons again. Congrats.
As for sizing, as i'm sure you know, there are several schools of thought re: road bike fit, plus there's lots of different possibilities for the fit needs of someone who is 6'4"; some guys are all legs, where other guys have a long torso, etc. Similarly, there are different design philosophies in terms of geometry, so a lot will depend on the specific bike. Generally speaking, though, a lot of ppl would think that 60cm is about right for someone 6'4". Personally, i'd shoot for a 62 or 63cm...
HTH
Rob