Have a 51 ltr in my wheel well as you say about 150-170 miles per tank, but floor needed no mod's
Last's me over a week, town driving, and over 200 miles on m/w that's over 2hr's driving, which is just right to stop for a break
Bigger is probably better but in an estate you are detracting from being able to use one of the biggest load space's you can get with the seats down

I'll not comment on the fitting of that

Dazzler, I looked into this in great depth before converting mine. Lowering the floor isn't really feasible as there's only about 20mm clearance between the wheel well and the petrol tank. I have an OMVL 74L, 4 hole tank (actually fills to 64L for some reason, but I'm told this is common with toroidal tanks) with a Stako Vapour box (for ease of fitting). I had to raise the floor by about 3" but, in the grand scheme of things, rarely notice it because I have a dog crate in the back!
Not pictures of mine but the same boot floor modification!
SDC12348 by
lazydocker, on Flickr
SDC12350 by
lazydocker, on Flickr
This was finished off with a carpet edge and a flap so that the boot looked like a factory fit

Edit: I get 230 miles between fills regularly, more on a run, with a heavy right foot and a lazy 'stat
