Well, (I will use here coordinates from 3DS max, will be easier to explain... and I will give you more extensive answer) the largest published campaign map is from Morning Sun and has size: X=-640,640, Y=-320,320 - so 1280x640. And this is also size of surface in 3DS Max on which is vanilla campaign map (but in vanilla map take only small part of free space). However, probably is possible to create larger map (for test I did map with size 1280x1280, but I didn't push it more).
But as you noticed, game not load models for settlements which are too far from coorodinate x=0 and y=0. It only vissual bug, but I didn't find way to fix it yet.
So, if you want to have all settlements models visible, then you can use space around x=-350,350 and similar for y (so, a bit more than half of avialable space in x coordinate and full space in y coordinate). Sadly I don't know more accurate coordinates, but in Last Alliance I have town in coordinates x=331 and still is visible.