Call your local Weil or Zoeller rep. They should be able to help you size your basin pretty quickly. They'll probably want to know your invert to the basin, the GPM of the pumps, duplex or simplex system, how long in minutes you expect the pumps to run, and what type of alarms/float switches that are required.

Thank you. I'll definately do that.

Order a copy of "Engineered Plumbing Design" by Alfred Steele;

you can order it thru ASPE