The minimum you want is 8 air exchanges per hour for national air quality standards but this may not cut it for removing steam from a shower. With a standard height 8' ceiling the minimum cfm basically computes out 1.1 times the square footage. I wouldn't be afraid to put in a 300 cfm cause you really want quick removal when it is used and a closed door will reduce the draw on a fan anyway. Get the quietest as possible or else it may not get used by some people. Lights are nice for extra reading light since most bathrooms are so dark anyway.
When installed, seal around it for air leakage by the drywall with caulk and/or foam sealant. Use an insulated hose to avoid condensation in the exhaust hose and support it so the hose slopes so that if any condensation does get into the hose, it will drain out.