A decent size pit is 24" across, as an 18" pit makes a duel pump setup very challenging. Pumps are made to run, and the constant cycling on and off that can be common in a pit that is too small is hard on the pump motor and wears out the switch.
In your case, I would suggest taking care of your gutters and downspouts, making sure that no water is being deposited within 10' of the foundation, and piped to the "downhill side" whenever feasible. As you mentioned, the grade surrounding the home should also be pitched away from the home. Taking care of these things can be a huge step towards keeping a dry basement while minimizing the need for a sump pump that runs overtime.