Warming PCB in cold environment using SMD components only
There are plenty of off the shelf surface mount resistors that can handle the power that is at your disposal.  You can even use multiple resistors to spread the heat and derate the parts.  For instance, many 1206 resistors are rated for 0.5W.  Using two of these to spread the heat would split the power between them, giving each plenty of headroom to ensure you don't approach they're specified limits.  A single 2512 resistor may be rated for 1W.  Refer to the manufacturer's part data sheet for specifics on each part.

Keep in mind that you're trying to transfer the thermal energy into the PCB, and into your sensor.  Depending on the type of heat sink that you're referring to, the heat sink may try to transfer that thermal energy into the surrounding air, which is what you want to avoid. Copper is a good thermal conductor, so you can use the traces that connect to the heating resistor/s to transfer the heat to the sensor.  If the resistor and sensor share a common node for direct copper connection, use that to your advantage.  Keep the resistor close to the sensor, and the copper between them as large as reasonable.