I've not found the site I was looking for that listed all tyre sizes with details of each size but I did find
this one. You type in the size your looking at and it calculates the O/D, width, etc etc........ for you. At least you can get the tape measure out and see what needs cutting.
I've run 31" on mine and now run 33" tyres and, as Bulli has said, the 33's do alter the gearing and you will be in 3rd to keep 30mph, 4th for 40mph and will have to change down to get up some hills. I sorted this by swapping the transfer box to the TD5 Defender's 1.410 ratio box and now runs like it was on a set of 205R16's again. :D
The transfer box way of changing the gearing is okay but it will mean that the low ratio is slightly over geared as this remains standard, to get both high and low back to normal you need to go with Bulli and change the axle diffs to 4.11:1.