"Some midfield players nowadays are 6ft 4in tall. Marc-Vivien Foe is 6ft 3ins, Vieira is 6ft 3ins, the boy Edu 6ft 2ins and there are a lot of players at Arsenal who are over 6ft," Ferguson told the Pink.
"It is not a thing you necessarily want to do something about immediately, but the long term view means you have to bear that in mind."
Ferguson dismissed any suggestion that he will put physical size ahead of skill but said some rivals can out-muscle United on the field.
"You only have to look at the size of our team compared physically to Arsenal and Liverpool," he added.
"You used to have creators in midfield, now you have got athletes and powerful people, but at the end of the day, I think ability is important.
"Ability and character will always be the pre-requisite of anybody coming to this club and we have got plenty of ability. I think a smaller player has to be exceptionally better than a big one because of the physical part,"