In other words, what map size would you say is 'too much' for the game to load after expanding and adding scenery? I've tried the max map size a while back, that's 'too much'.

That honestly depends on computer strength and how much detail you are going for.

that all depends on how much scenery you put in. game saves start to mess up around 12mb

I'm a sucker for detail and scenery, help narrow it down?

Usually the normal sandbox filled most of the way (with high detail) will start overloading the average computer.