I wouldn't just go on the galaxy size for the number of A.I. you should use. The higher the A.i settings you use IMO the less A.i. you should have as the game gets unstable in my experience when you have loads of godlike A.i. on the map.
?? What do you mean? I am currently playing a game with 80 (approx) AI on an insane map. The game has been very stable, hasn't crashed at all. Now about 500 turns into the game I'm down to about half that as factions have been killed off. However, the game has been fun and I see no justification for your comment. If you are having issues I would recommend you create a support ticket.
On the original posters question, this is what I'd recommend.
For a full map of factions.
8 - tiny
15 - small
25 - medium
40 - large
50 - huge
65 - gigantic
75 - immense
90 - excessive
100 - insane
For an empty map for you to explore.
2 - tiny
3 - small
4 - medium
5 - large
6 - huge
7 - gigantic
8 - immense
9 - excessive
10 - insane