I'm running 225/70/15 General Grabber AT tyres. The just fit in the arches with about 5mm to spare between tyre and rear bumper. Plenty of clearance above and don't fowl on full lock, using the calculator in the link below you will see whats close.

http://tire-size-conversion.com/tire-size-comparison/

I just done it and you could fit 205/70/16 tyres. My set up is 27.4 inch diameter, 205/70/16 would give 27.3 inch diameter. Have a play about with the calculator it will give you good idea. Personally I wouldn't go any bigger that what I've already got.

Good luck,

Phil

 

Not sure if you would call 60mm "slightly lowered"? Maybe compared to vans that get slammed to the floor?
I am running 215/65/16 AT Tyres (which I think is a 27" diameter?) and the clearance between the tyre and the bumper rear and the arch front was as close as I would personally want to go.
With the lowering springs that were fitted (I would guess 60mm), I was not happy at all with the clearance to the top arch any kind of suspension bounce and it could easily foul - and I wanted the AT tyres to be able to drive in grass fields,etc. similar to your description, so you will need that travel I would think. I've reverted to standard springs (don't like the lowered look anyway)

Search the forum for "off-road inspired" to see a big thread on AT Tyres T:

 

Thanks guys, will look at that thread. Tbh I have no idea how much my van is lowered from 'standard': with it being ex-AA is was sky high when I bought it due to all the AA equipment being removed. I've lowered the rear approx 100mm and the front approx 80mm from AA height; it looks much better but doesn't look 'low' at all- more like how you'd expect a van to look normally.
On a side note, if anyone has measurements from the ground (or centre of hub) to wheel arches for whatever 'standard' height is, I'd be very grateful as it'd help me get a benchmark.