There is no way to set the pop in single player. It has to remain at 50, cause the computer is programmed to play at 50 pop limit. There are a few ways to kinda "cheat the system" however...
goto multiplayer, serial game, click create game, when you get a little box with some modem information in the corner just click ok. Now you are at the game setup screen, just add the comp players and set the pop limit in the settings screen. Warning however the computer is not designed to play at a higher pop limit so they may do some weird stuff...