There are couple of parameters that you should look.
1) Rdcon. This parameter tells how much energy will be burned in device while FET is conducting current. Low is better and range of 10-20mOhm parts are quite inexpensive.
2) Vdss is enough high. Well above your working voltages
3) Id @ Vgs=5V is high enough. FET conduct current at used source voltage.

I don't know how small FET you are looking for but here is one example from RS catalog
IRFSL3806PBF