How to calullate RMS current through switch (MOSFET) in boost converter?
The RMS current is the equivalent DC current, so if you switch 1 Amp of current with a 50% duty cycle, your RMS current would be 707mA, 

However this is not the only part that causes power dissipation in mosfets, there is also switching losses, to do with how when the mosfet is switching on or off, there is a breif moment each time when its not fully on or off, but rather acting as a resistor,

Boost converters depending on there topology may also see some heating through the use of its body diode instead of an external diode, but this is rarer these days.