Before you launch a league, you should (it is highly and strongly recommended) set a maximum number of players allowed on a standard roster. To set the maximum number of players on a roster you will need to go to the "Set-Up" area of the league and then select the "Registration" tab. Set your roster limit and save. Your league is all set!
Recommendation: Make sure your captains are aware of your roster limits. If they want to add additional players to their roster, you can implement an additional roster slot fee.
Note: Once you have created a sport specific league setting that works for you, copying these settings will save you a ton of time and keep everything consistent. This practice is recommended.