I think we are at that moment in time, when we have to admit, that there is no limit.
I think for the people that use their phone as a phone, then 2GB is fine, but if you use it for other stuff then 4GB is looking to be a bit limiting.
I have a fair number of top end phones and in piddling about with them, I have filled them up with apps for no reason other than bugger it I can ( I use 128GB SD Cards for my main Phones and 64GB for the non important ones ) and I just plonk about swapping from Phone to Card for the hell of it, just to try cramming more stupid junk that I will in all likely hood, NEVER use, but there you go.
I have filled up phones to the brink to force them to trundle to a halt purely cos Im bored and its surprisingly easy to find limits on 3 and 4GB phones.
Of course what you out on the Card, whether internal or MicroSD, does not in itself use up any of the Phones RAM, but each app does take up some of it of course, but I think that if you have some kind of RAM cleaner thats running in the back ground, that should help somewhat?
I have one that pops its head up every so often to tell me stuff that I dont give a monkeys about, and it does seem to be doing its job, however I am also sure that its lying to me because I seem to have saved terrabytes of RAM in the few months that I have used my S7 LOL