From the information you give, the thermal resistance of the transistor is 37 degree C per watt from the junction to the mounting base. You say the maximum junction temperature is 200 degree C but I think this is a 'storage temperature'. It is more likely that the operating junction temperature is around 150 degree C. The 37 degree C per watt is based on a maximum junction temp of 150 deg C.
If you want to run the transistor with a 2 watt dissipation in an ambient air temperature of 40 degree C, then the thermal resistance from junction to ambient will be 150-40=110 degree per 2 watt which works out to 55 degree C per watt. The thermal resistance of the device is 37 deg C /watt so you need to have a maximum of 55-37 = 18 degree C per watt as the thermal resistance of the heat sink. This is a small heatsink as heat sinks go.