Umm, hold on a second....

The "Map size" in the Light parameters refers to the Shadow Map size. This is only relevant if the lights are using Depth Map shadows. It might be what they are referring to - I don't know this light set or what type of shadows it uses.

But if the lights are using image maps, they might mean the "Image_Resolution" dial in the Light node (click on the Advanced material properties button shown above).

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)