Ok, I don't imagine he has a bike, but I recently read on article on his body type and I thought that no way would this guy be able to just be given a standard issue 62 cm bike. So, what would Phelp's bike look like?
I don't have the specific measurements, but:
Leg inseam equivalent of a 6' tall man
Torso of a 6'8" man
Arm length 6'7"
Feet size fourteen
"dinner plate" size hands.
So build his bike. Assuming traditional geometry, how about
Top tube size
Seat tube size
crank arm length
and, since he's millionaire, how about outfitting his bike with the best components and best frame. So pick a framebuilder too, as well as frame material and type of lugs.