charging system

what's the largest battery our charging system is capable of comfortably handling..? i'm currently running off a 38Ah but at times it feels a little 'short' if you know what i mean.. :|

as above, what's the easiest method..? of course it has to be from underneath the car given how cramped the rear side of the engine is.. but if you are alone and have no help, how do you go about it..? unlike the power steering pump which has some sort of hole in which i usually insert a bar to...