Hi,

The power dissipated in a MOSFET is the voltage across it times
the current going through it.

The voltage across the MOSFET when it is on will be the
resistance of the MOSFET, Rds(on) times the current going
thorough it.

A MOSFET IRLR/U2905 has a on-resistance of 0.027Ω.

Power dissipated by the MOSFET case when you have 2 Amps of
current going thorough it will be: (2A = 100 LED's 20mA).

P = 2A^2 * 0.027Ω = 0.108 Watts.

(No problem without heatsink).

Best regards,

Luciano