look at the I/V characteristic curves for the MOSFET. they can be found in the datasheet. You aren't going to want to use the MOSFET in the active (saturation) region if you are trying to switch a large load, because it will dissipate a lot of power. You will want to use it in the ohmic region, which will give you a much lower Vds which corresponds to lower power dissipation (P=Vds*Id)
Look on the I/V curves for the device you are using. find the current level you need to drive the motor, and look at the curve for the Vgs you intend to use. if the current level falls in the ohmic region (ie- the part before the line goes semi-horizontal) then you will be OK. If not, you will either need to choose a device in which it does, or increase the voltage with which you drive the gate. the higher the Vgs you use, the lower the value of Vds for a particular current, and thus the lower the power dissipation in the MOSFET.
as long as you aren't trying to drive a huge motor, you shouldn't have too much trouble finding a mosfet that can drive a few amps in the ohmic/triode region at a gate voltage of around 4.5 or 5v as you will get from a PIC. Try looking at the IRF540. that can give you around 10 amps with a gate voltage of 4.5v, with a Vgs of only 1v, meaning only 10 watts of power, and since the device is rated for 130W of power dissipation (with proper heatsinking of course) you should be fine. If you need more than 10 amps, then you can use a separate power source and another transistor (or an optoisolator) to switch the MOSFET gate at a higher voltage from the PIC.