difference in TT bike vs road bike

not an easy question to answer at all... so many variables (between bike companies themselves as well and their own "sizing")

The best answer, is get fitted first. Preferably from somebody who works with triathletes. Ask for your dimensions... Stack/Reach, etc. ST has a comprehensive guide on the stack/reach of most bikes out there now, that's a place to start.

Competitivecyclist has a fit calculator on their site for free... it's another place to go to learn a bit more.
Things can change depending on how flexible you are (and how steep you want/like to ride)  what length cranks you want, whether you want a longer or shorter stem, how long your aerobars are going to be... etc.

I hear a rule of thumb that a tri bike is typically a size smaller than a road bike...  For me, that seems to be the case.  Professionally fit on both and my TT is 61cm and my road is 63cm.

But that's just the beginning.  each bike will have slightly different measurements.  Meaning, all 61cm bikes are not the same.  My 61cm Cervelo is better for me than the 63cm Cannondale...

for me i am a size 48 on my road bike just purchased a new tri bike size 46-1/2, 
good luck

Sarah