Single cab Canter, 4.5 tonnes, 4mx2.1m box, permanently made bed motorised drops down from ceiling, seating round table for eight, shower/loo 1.2x0.7m, 240 litres water, 240 litres diesel, all solar/electric, drop-top to fit in hi-top container, 3+ years continuous occupation through 34 countries. Clearance and entry/departure angles more important than 4wd.
Really depends where travelling in which countries. Our box width is 2.1m, to fit in container but also Europe (we are based in Aus). A nice addition would be electric retractable mirrors but we can pull in the mirrors we have. Had to visit the "looking glass wallah" in India/Nepal. The loss of a few cm can be the difference between forwards or backwards. 15-point turn in busy, narrow, Chinese street full of traffic when gps took us to a 2m height barrier - once turned round we followed a cab. Mongolia, Morocco, Tibet, size didn't matter much, driving technique more important.
Just an observation - some larger vehicles we met used the extra space for toys and had poor ventilation.