The power dissipation can be broken down into
1) conduction losses
2) switching losses
You can find the conduction losses by a simple IRMS^2 * RDSon where IRMS is calculated by the PIXls spreadsheet and PI Expert application and RDSon can be obtained from the datasheets
You can find switching losses by plotting switching voltage and current and estimating the energy loss per cycle.
Once you have an estimate for the power loss you can use this value to choose a heatsink
Most manufacturers will include curves for Power loss vs temperature rise for a given package (for example the TO220 packages have a fixed thermal resistance junction to case)
Attaching once such curve from Aavid-Thermalloy for a TO220 heatsink