Without a cut sheet or installation instructions to work with, even roughing in the hand dryers is going to be 'iffy'.
The knockout might be anywhere on the baseplate of the unit. If tile is used as the dryer's mounting surface the problem's will be compounded for sure.
It would be good to ask the supplier of the hand dryer to know the manufacture and model # of the unit. They're website's bound to have the information you need.
If it has to be ADA compliant, then the fan's air outlet must generally be between 38" and 48" above the finished floor.
You may want to check local codes
