I seem to remember someone mentioning a theory in Index that Touma's power had to do with removing the 'quantum uncertainty' that ESPers create/manipulate in order to manifest abilities that should otherwise be impossible for a person?

If this is indeed the case then there may not be an upper limit on his power as the power would just be forcing the the universe around his hand to have its 'natural' uncertain state as opposed to any 'unnatural' uncertain state any ESPer powers (and maybe magic too) induced to produce their effects. The only limitation I can think of in this scenario would be a time delay between an unnatural effect entering the area of influence of Touma's hand and the uncertainty state that makes up the effect being returned to 'normal'. That being said as long as the time delay is less than the distance light would have to travel between the edge of the area of effect and Touma's hand he shouldn't have anything to worry about.