going to stick with the 14" wheels and fit van tyres 185/75/14. That would be the max radius at 634mm for a doblo. Somebody else here had his doblo with radius of 634mm on a 195mm.
The idea is that the extra large profile will make the vehicle a bit more pothole resistant (even been to glasgow ?) and those kind of tyres as standard are far better with carrying heavy loads, my doblo being pretty loaded to the max.
Its almost a doblo 4 by 4 now. large profile tyres and raised suspension all round.