Burner size is more about how fast you want your wort to come to a boil in my opinion. My Sq10 style cup burner with a 5 PSI regulator from my turkey fryer, will boil 10 gallons over the side with no problem. I don't much care how fast the water comes to a boil to be honest. Using three pounds of propane, I can boil my mash water, my sparge water and boil 8 + gallons of wort.
EDIT: Let me rephrase that, the more liquid you have, the more BTU's you will need to bring it to a boil, respectively speaking. You can get the biggest, baddest burner out there, but at some point the bigger burner just wastes gas. A given pot of liquid can only absorb a given amount of heat, in a given amount of time, no matter what. Bigger burners use more fuel even at a low setting, as their low is (possibly) a smaller burner's output wide open. Gas volume and gas pressure at some point, become inverse curves
