You'll pick the delta-P across the control valve. The higher the pressure drop that you can take, the smaller the valve can be. You're dealing with a very small flow anyway, though.
A steam trap is just an automatic valve. It'll open when it senses air or water, and close when it senses steam. I'm assuming that you're looking to size a temperature control valve on the steam inlet to maintain a given temperature on whatever it is you're heating. This means that the temperature control valve will throttle the steam flow based on the temp it senses. At lower loads, this will give you the same effect as a pressure reducing valve. As the TCV throttles-in, the steam pressure in the HX will fall. You need to select a steam trap with this in mind, and make sure that the condensate can drain by gravity from the HX. Companies like Spirax Sarco, Armstrong, etc can help you with this application.
