Alternator size isn't a problem. If you have a 'small' alternator it just takes longer to charge the batteries so, the more batteries you have to charge, the longer it takes. The problem with relying on alternators is that they never charge to the battery's capacity. In short they never charge a battery so that it's full. That's where 'smart' chargers like Sterling Battery-to- battery chargers score.

As for battery size, it really depends on the load of your fridge etc. and how often you'll run it when compared with how often you'll charge it. If your fridge draws 2 amps/hr, with a 110 amp/hr battery it'll go flat in 55 hrs theoretically. Of course the battery will never be fully charged and the battery won't be able to give 2 amp/hr until it dies but you get my point.

For what it's worth, my Indel compressor fridge ran virtually continuously (in W Africa) simply charged off a 120W solar panel into a 110 A/hr battery.

Q

X2 on the alternator ability... To be honest from what I have researched Optimas would actually be pretty far down the bang for buck rating. As far as cost effectiveness goes the best would simply be normal wet cell batteries - What would make sense would maybe be a normal, large good quality starter battery and then a very large deep cycle (or even normal - if it is very large then deep discharging won't be such an issue). In my defender I had something like a 200a/hr battery off a bulldozer - but it was HUGE in size and weight! There is no reason that I can see why you could not put an intelligent charger (ctek etc) after the NL split charge to make more effective use of the split charge. Or upgrade the alternator to 'intelligent charging' Check Sterling Power Products regarding this. If you can carry it and look after it then I would argue that a very large conventional wet cell would work as well as a smaller fancy technology battery and probably cheaper! I seem to remember you were talking about winches Darren? That would also be an important issue power wise...
Like you I am also at the 'experimental' stage, so will post up info on power and batteries and consumptions as I am expecting a small DC clamp meter this week to enable some proper testing!
G

I have read that deep cycle batteries don't like being discharged quickly - which is what a winch does so might be worth researching a little.