Neither Silicon nor I know the exact limit, but at higher resolutions it would just refuse to compile. Precipice's lightmap size is 16,337 kB, and Silicon said his method involved lower it from a size that gave the "Lightmap size too large" error in Tool to a size that wouldn't produce that error. I'm guessing that means that the size limit he is referring to isn't much higher than that. He uses 2048x2048 in all the major areas and 512x512 in less important places on that map.