Slitherine

Apparently the 'Huge' & 'Gigantic' galaxy sizes are hardcoded, at least I could not find them in the 'Globals.lua' file.
That being said, I found increasing the 'GalaxySizeFactor' to a +2 still worked up to 'Huge' but not on 'Gigantic'
Could be a 'pc' thing or just not doable, thought I would address this.

Editing the 'Globals.lua' file per the Galaxy Size Factor is interesting to try.
Changing: -- Galaxy
GalaxySizeFactor = 1, to a 2 for example (tried 4 & the game would not load) increases the size of the galaxy.
Based on the way the number of Star Systems are assigned per Galaxy this allows a Tiny Galaxy, for example, to be more spacious.
Or so it appears.
One can even adjust the minimum/maximum distances between these Star Systems, but beware of traveling time in the early game.

It appears GalaxySizeFactor = 3 is the max.
But it really is huge...even on 'Tiny'.

@eddieballgame
Looks like I will need to keep tabs of your posts to learn more about modding this game.

Is there a sub forum here for mods?

Not that I am aware of.
With the new 1.01.05 patch, I need to revisit my mods.