It depends on what the goal is in feeding. If you are general purpose feeding (goal to help the BG grow) then the size area depends on how many fish/and what % of their diet comes from pellets. In that situation you want the pellets to spread out enough so that all get to feed. A typical feeder covers a good area for that purpose. One method I use is to let the feeder go off and about 20 seconds later hand throw some food in shallow protected place for small fish to eat.

If you are feeding small pellets for new/yoy fish then feed over areas with thick cover (xmas trees)so that they are protected.