It depends upon how you intend to use it - I have fitted a Webasto Air Top Evo 40 4 Kw to my LWB high top van. BUT I use a heater for rapid heating of the van after being out with the dogs and for rapid defrosting in the frosty mornings and never use the heater for overnight heating ( that is what bedding is for ). As I have a workshop with a couple of dozen heaters in ranging from 2Kw to 5.5Kw I can swap a different one in or out as I wish.
If you are intending to use it as overnight background heating the 2Kw is the one to go for.
A 2Kw Webasto will run at 0.9Kw at lowest output and a 4Kw will run at 1.5Kw at lowest output so you have to take this into consideration also if you want to run overnight continuously.
A 5 or 5.5Kw heater is far too powerful for all T6 vans unless your useage is Northern Finland in the winter months!
OEM fitment to T4/T5/T6 vans was a 3.5Kw heater - the closest to this heater was the Webasto Air Top Evo 3900 3.9Kw heater which superseded the 3.5Kw ST model. The 3900 ran at 3.5 Kw if boost for the extra 400W was not fitted.