I just used a 20" paella pan to make a large paella for 10 people. The pan covers 4 burners. The problem that I had was that the center of the pan never reached the temperature of the parts of the pan under the burners. If I raise the gas levels enough for the center to get to a boil, the outside areas get too hot and burn. Difficult to make an even paella. Any suggestions? I have looked for, but not found any heat diffusers that are large enough for this pan.
The method I use is to start the paella pan on the middle of the biggest burner, then once the rice and liquid is in I move the pan over the burner every 2 minutes to make sure it cooks evenly. For example I'd move the pan so the left side is over the burner first, then move it to the right, then top and then bottom, keeping with that pattern.
The oven can work, however, if you have the option, I would suggest outdoors over fire. Paella is a festive occasion, and this adds to the spectacle. For your 20 inch pan, you could construct a temporary brick fire pit sort of like the one below, as a friend of mine regularly does. Alternately, you can place your pan over a charcoal grill, provided it fits. Mine, for example, fits into the opening of my Big Green Egg smoker/grill. If you don't have those options, using the stove top will mean a constant rotation of the pan. That gets messy and potentially dangerous once liquid is added. So, I would agree with the oven recommendation...if it fits.
A large paella would generally be cooked in an oven, possibly with some extra cooking on a burner at the end to brown the rice at the bottom.