How can I achieve an even heat across a large paella pan?
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.