Way back when, the rear tyres were always 3.25 or 3.50 on the WM2 rim.
Judging from the swing arm in my '56 frame there's plenty of room (the tyre is wider than the rim so the tyre width would be the limiting factor - looks like a WM3 rim might give the option of a better tyre - 4.10? - assuming you can get a front one that'll work with it)
I find the specs for the tyres quite difficult to work out; there are max and min widths as well as 'recommended' and some of the specs given by rim suppliers (Hagon?) can be interpreted differently to the specs from the tyre manufacturers (Avon give the max and min for their tyres as well as the 'recommended'). I had loads of fun trying to work out what my modern tyre options were for my WM2 rims. (I've settled on a 3.25x19 Roadrider on the rear, it looks very narrow but according to the Avon specs it's appreciably wider than the 3.50x19 SM Mk2).
To move off topic slightly, I wonder who you're getting the rim from (and are you going for chrome)?
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)