Hi sgp,
You’ll want to have a look at our troubleshooting wiki for lighting here: A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums
For the specific issues you’ve run into look at these sections.
Lightmap Resolution: A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums
Shadow Seams/Shading Differences with indirect lighting:
“I’ve always thought that the resolution should be whatever is lowest that can fit in all the islands with padding and etc, but am I misunderstanding lightmaps in that I should be giving large areas a larger lightmap, no matter how simple they are?”
Lightmap resolution is dependent on the efficient use of the 0,1 UV space when packing the lightmap UV along with the size of the mesh in the scene. Having a large mesh with a low lightmap resolution will generate low quality shadowing. The idea is to keep the lightmap resolution relative to other surfaces to make sure to keep it low enough for your needs and quality bar so that the lightmap density is consistent for the scene. Otherwise, it can break the consistency of the bake. Use the Lightmap Density view mode to check this.
-Tim