Building on what I wrote and putting it somewhere more appropriate:
The idea is that the game principles seem to work best when the number of Spies is equal to the number of mission needed to win.
2 Spies => best of 3
3 Spies => best of 5
4 Spies => best of 7
5 Spies => best of 9
etc.
Now, we also know that Spies need to be in the minority. Thus, if we have an even amount of players, we can't split it down the middle but need to offset by one:
6 players => 4R/2S
8 players => 5R/3S
10 players => 6R/4S
12 players => 7R/5S
etc.
So for even numbers, we can go from number of players to number of spies to number of missions.
What about odd amounts? Each of them can be lumped with either the even number below it or the one above it.
For example, 7 players would either be 5R/2S or 4R/3S. Or, in other words, there's either 3 more resistance than spies, or 1 more.
I believe both options are playable, though maybe not necessarily with the same rules. If there's three more resistance, then the Spies are in an underdog position with less options to sway the votes. I feel that they need the advantage of full information to balance things out, so in such a setup I would give them all the names of their spymates. If there's only one more resistance than spies, though, I would not give out the full list of spies but just one name.
Or, you know, we could simply not play with odd amounts of players...
Working out a set of rules for the team sizes per mission is a bit more complex, but I think it's feasible.