Help Choosing a mosfet
In this answer I explain the most relevant parameters when choosing a FET, and use the FDC855N as an example. Like asker of that question you have literally thousands of FETs that fit the bill, like the FDC855N, for example:

\$I_D\$ max: 6.1 A
\$V_{DS}\$ max: 30 V
\$R_{DS(ON)}\$ max: 27 m&Omega; at 10 V  

The low \$R_{DS(ON)}\$ means that power dissipation will only be 100 mW.  

For heating you don't need a high PWM frequency, so switching losses can also be kept low.