A whole floor is about as efficient as a single modular piece, so even if it has to be loaded more often it’s not much of a hit. The only limitation is due to the lightmap resolution so if you aren’t getting enough detail in the lightmap then you would need to split it up into pieces so that you can use multiple lightmaps.
The lightmass importance volume allows you to focus the quality of the lightmass on the most important parts of the level, so ideally you would keep it over the smallest area.