Heat Sink for SOT223
Take a look at this publication from Texas Instruments (you can go directly to pages 12+ for a quick look) : http://www.ti.com/lit/ml/slup230/slup230.pdf

Someone made an online spreadsheet calculator about it : http://circuitcalculator.com/wordpress/2007/02/16/pcb-thermal-copper-area/

You can also read this Application Note from Fairchild about "Maximum Power Enhancement Techniques for SOT-223 Power MOSFETs", which seems to be exactly what you're looking for :)
http://www.fairchildsemi.com/an/AN/AN-1028.pdf

As far I as know, there is no heatsink for SOT223 packages, the only way to dissipate the power is through copper via pin 4.

I doubt you will be able to use your mosfet at full power (1.5W) and at 100°C+ when the Tj max is 150°C, unless you have a quite big copper area and a cooling fan.