Only you can decide how much/often you actually use the full capacity of the boot. It might be worth taking everything out and "only" putting in what you NEED for each day for a week or so .. just to see. When I had the full size tank fitted I was a tad concerned .... but now I realise I was actualy very lazy and just left a load of crap in the boot from day-to-day. I now "manage" the boot far better, and don't leave rubbish in all the time.
The only times I really notice the difference are ..
1) Trip to the dump ... now have to use the trailer, but how many times do I go there ... once every couple of months ??
2) Trip to CostCo shopping .. its a fair distance and only worth doing for a "big" shop, so we end up with a fair bit of stuff on the back seat.
3) Main holiday to France...no room for caravan awning and loads of other kit, okies .. it could go inside the 'van but ... As we take bikes on a roof rack .. I've bought a 1/2 width roof box which is bloody huge !!! and last year found a major unexpected advantage... had to pack up in the wet .. so before the boot would have got soaked from the wet awning, or the inside of the van ... but the roof box just didn't care .. and it drained as we drove !!
So what am I saying in a long winded way ?? .. There are ways around the loss of space, and even 1/2 a miggy boot is way bigger than some other full size boots ... I have no regrets at all on having the LPG with an 80 litre tank .. 
Well, I can honestly say that I could easily have managed if the boot was glued shut for the past month!
I guess it would stop me collecting crap! 
I’m kinda thinking that if I’m going to do it, a should just get on with it and have done! I use the Mig to drive straight across town centre and back everyday in rush hour, and don’t think a spare wheel tank would last very long. Also there’s only one LPG station I can think of in York (at Asda), and its not really on route. (not a million miles away like either!).
I’m already happy with holidays, I have ample space in the caravan for everything. And will probably only be 3 people in the car at the most anyway, so theres always back seats.
I have a small diy trailer too, so if I’m stuck that’s always an option.
And I’ve also remembered that I bought some roof bars about a year ago which I haven’t used yet – so I can always sling stuff ‘upstairs’ 
Hmmm.... LPG does make more sence the more I think about it 
I'm coming round to it, too-but really do need the boot space and spare wheel....
I suppose I could lose the spare and stick a foam can in the first aid kit compartment and use the breakdwon cover if needed.
Hmmm.... :-/
space aside, i think biggest is best when it comes to economy, the bigger your tank the least likely you are to run out before finding another station, esp on holiday or in unfamiliar territory, or just getting to your usual.
About once a month have to stick £20 of petrol in, partly due to the start up on petrol thing which is unavoidable obviously, but also because i run out of lpg now and then and have to do part of a work journey on petrol, maybe others with bigger tanks do the same when knowing they/we have petrol to fall back on, maybe not, but i am sure i would use less petrol if i had a bigger lpg tank....therefore bigger is cheaper.... to my thinking anyway.
But in my case, re space, donut it is and thats that.....but would always encourage the biggest tank possible

re spare wheel, its in the garage! Boots empty, ment to get a can of tyre weld, didnt get round to it....
Fitted new tyres a month or so ago, you guessed it, nail in the tyre by the time i got home. Left it to the morning and drove back to the tyre place and they fixed it. Tyre lost 3 psi in about 14 hours.
Now i dont want to tempt fate again, but when did you last see a puncture? in my experience with a nail or screw,it dont go down straight away. I check my pressures regular, and do tend to notice if one is low when driving. I once had a screw in a bike tyre for over a week, no money to repair it. Just kept pumping it up.

But your right of course, always best to have the spare obviously.
I did have the spare in the boot for a while, never noticed it was there. Being rubber it doesn't slide about. Suppose i should put it back in....
