I am baking a large scale office interior and I get the error about too large lightmap resolution. I tried to add some slices to avoid too big polygons but that doesn’t resolve the problem. Do I have to split the big meshes into smaller ones or is there a more elegant way to deal with it?
Is it an option for you to use custom (lower) lightmap resolution for large surfaces? Maybe you could lower the resolution for ceilings and walls.
As @tim said, you may try lowering the lightmap resolution for the largest objects.
And if it is not enough then yes: you will have to split the big meshes into smaller ones. When meshes are unwrapped and assigned to lightmaps each mesh must fit entirely into one lightmap. So, the error you mention means there is some mesh that cannot be unwrapped to one lightmap.
I wanted to avoid lowering lightmap resolution to maintain quality. I found the problem and it was just one mesh - not the biggest by any means. Surely it is a bug. I will send it to you @wojtek by email. Maybe you will be able to find out what was wrong. I ended spliting this one mesh. The other much bigger ones don’t cause this problem.