If you want big big big, go for the Iveco Daily LWB H3. Common miss-conception, but there is no XLWB Iveco, the MWB is as big as a LWB transit, the LWB Daily is huge with a 4.5m loadspace and 2 metre rear overhang which means the wheel arches don't get in the way if you want a rear bathroom or garage. Sides are straighter so they are very popular as a campervan as the walls do not close in at head height, I have one myself

You can also get a standard mattress sideways across the Iveco Daily at its widest point (about 600mm off the floor). Very few vans can manage that, the Fiat Ducato/Citroen Relay are one of the other that can, but they are much smaller and only fwd.
The H2 roof is most common and headroom inside is 6'3", the H3 is taller and has closer to 7' of headroom. If you need a poptop on that you are mad

35s12 - 3.5 ton, s = single rear wheel, 12 = approx 120bhp. 50c18 - 5 ton, c = dual rear wheel, 18 = approx 180bhp

There are also 29, 45 and 65 versions.
They also excel at towing, after 2004 a single rear wheel will tow 2.8t, or 3t with the 3.0 engine, and dual rear wheel models (c) will tow 3.5t if you have the 3.0 engine

This is my MWB (1 metre shorter than the LWB) before I converted it...really wanted a Transit but despite searching I couldn't find one of the rare versions that had a high enough towing capacity.

Part converted:

Useful links:
http://www.sbmcc.co.uk (self build campervan forum)
http://z6.invisionfree.com/Iveco_Daily