Right now there are some useful controls over effective galaxy size. Of course, there is the size option itself. Then the user's ability to select relative frequencies of stars, planets, habitable planets, etc. allows the user to vary the effective size. So far so good.
My suggestion...
To complete this functionality allow the user to select the cost/time to develop range related techs such as "life support" without affecting the rest of the tech trees. With this addition, one could make a gigo-normous effective galaxy by making range techs very costly and/or time consuming, while playing in an gigantic galaxy with, say, abundant frequencies.
I would enjoy this very much (I think) since I take a kind of rpg approch to the game and this would better represent the structure of the galaxy and add much excitement to a new encounter.
This is probably an easy mod, but I'm not a modder. Also, I think that this should be considered for addition of the nest update currently in beta.
Take care.
Cornfusion