What is the average height for a WSB/MOTOGP rider?

No average really.

Dani Pedrosa & Marco Melandri are both tiny, just over 5ft yet Rossi & Camier are 6ft 2".

Dani Pedrosa Height

Don't forget Loris "Capirex" Capirossi. He too is a small rider.

I recall a photo from the Ducati website wherein some members of the team had to assist him to get on his Ducati Desmosedici in 2003 (or 2004, I think...)

He needs to be tall enough to reach the track on the start line to balance the bike but short enough to be able to tuck right into the fairing to reduce wind drag, They need to be strong enough to man handle a race bike at the bikes limits for up to about 50 minutes without fatigue, but weigh as little as possible and carry no body fat at all... To specifically and exactly answer your question there would not be any of those riders that would have all the data needed to get the answer, they have more important issues that need their attention....