Alright. Time for more empirical testing.
All settings Abundant/Abundant/Abundant/Rare (Stars, Planets, Habitable, Extreme) on a Scattered Map with 7 opponents.
NOTE: FOLLOWING ANALYSIS DOES NOT INCLUDE THE PLANETS HELD BY MINOR RACES. OOPS. SO TREAT AS A ROUGH GUIDELINE BUT STILL MORE OR LESS ACCURATE - SEE REPLY #48 FOR UPDATED ANALYSIS
Tiny: 25-29 total colonizable planets (two tests)
Small: 40-45 total colonizable planets (two tests)
Medium: 78-87 total colonizable planets (three tests)
Large: 105-127 total colonizable planets (two tests)
Huge: 134-157 total colonizable planets (two tests)
Gigantic: 174-177 total colonizable planets (two tests)
Immense: 215 total colonizable planets (two tests)
Excessive: 215 total colonizable planets (two tests)
Insane: 215 total colonizable planets (two tests)
The main problem here is the map size grows much faster than the number of planets. It seems like it is at around Gigantic when the map size starts to outstrip the number of planets. Just by a look at the minimap, at least. And it flat out stops growing at Immense.
That actually sounds like a bug to me.
Just by glancing things seem "reasonable" up to either Large or Huge. Depending on preference. But once it hits Huge, the problems start. Just from eyeballing it at least.