Pouyan,
The method I usually use when sizing equipment (radiators or otherwise) is to set each room of the building up with a set point temperature and an automatic heating type in TRNBuild. I then find the max hourly heating required from among the Type56 outputs and then size each piece of equipment accordingly. It sounds as though your method is the same.
10C sounds correct for a design air temperature but keep in mind that the radiator's capacity can be quite different if you are asking it to maintain an air temperature that is significantly different than the design air temperature. You may run into trouble if you set the design air temperature to 10C but are in fact trying to maintain 20C in the room.
I unfortunately don't have any experience regarding setting the number of channels. However, I would encourage you to play around with the value. Make your best guess and run a simulation. Increase the number by 20% and run again. Decrease it by 20% and run it again. Look at the results each time and determine whether those results are sensitive to the value in question. If they are then you can dig in and try and get a more accurate value; perhaps by talking to manufacturers. If they aren't then you can proceed using your best guess value.
kind regards,
David