Sorry let me back up. Basically if you are trying to capture any indirect lighting detail then what you have isn’t going to be able to capture much. You can use what you have, it will work fine, it just won’t capture much detail for things like indirect shadows.
Personally I prefer something toward the yellow-green region. The green color covers a pretty large range of density values, you could likely increase your lightmap res 20-30% and it would still be green.
The squares indicate how big the texels of the lightmap will be, you can think of that as the resolution of lighting detail you’re going to get.
I’m still not clear on what exactly the problem you’re having is though, but the first thing you should probably try is to do a lighting build on production rather than medium.