Are there any general rules on when a large world should be broken up into tiles? If not, I would like to get opinions on this. I have used tiling before, with very large layouts, but now I am creating a smaller layout with World Machine – with a 10km x 10km map with 8068 final relolution.
It always depends on the game and what kind of assets you use -> e.g tri count, texture resolution, quality of LOD’s, mesh density,… 
Most of the scene is outdoors, so foliage and trees are my biggest concern. The grass is low poly, but a lot of it. The trees (speedtree) average about 1500 tri count.
Then I would split up your 10x10km map, otherwise you will get performance problems -> also make sure to add hills and other stuff that blocks the view of the player + use culling volumes + good LOD’s
I find using 1km-2km tiles is the sweet spot. You aren’t loading too much in one go.
I also find that using 500m tiles tends to slow things down when you have lots of tiles.
Bare in mind that you can still stream your meshes in separately from your landmass. So you could use 2km landscape tiles and 1km levels.
Thanks for the info guys, this is what I was looking for. It gives me a baseline to do do some testing… thanks.