What is the general rule for map sizing that keeps map loops nice and tidy? For instance, if I built along the split/middle section of the map where the loop begins and ends, how do I get to so that bunker objects don't overlap unevenly at this spot?

If I recall the rule was always make your map sizes divisible by 64 if you don't want terrain objects to overlap, but I'm not certain if that's it. I haven't made custom terrain in a while ever since I accidentally deleted a lot of my older stuff.

Make the map dimensions divisible by 24.

Thanks. I thought I had that number wrong.

12 if you want to be really precise, but usually multiples of 24 is best.

Height doesn't matter as much, but width it does count for.

Unless you do y-wrap, which only one decent map in existence has; and that map's only really decent for novelty.