replaceing a toggle switch for EXHAUST fan for small bathroom with a rocker switch
A simple on/off switch is best for a fan. This is a 'single-pole' switch. For a rocker style, something like this would work

http://www.homedepot.com/p/Leviton-Decora-15-Amp-Single-Pole-AC-Quiet-Switch-White-R72-05601-2WS/100058788

The switch will most likely be rated for 15 amps. This is fine, the switch must be the same as or more than the fan.