Do you mean in the options before you atart the campaign? Small/Normal/Large/Huge ?

Or Lets say you chose Large and in game you want to "overpower" a unit.

It can be done like this -
Lets say you have 4 Hastati understrength and you are about to retrain them in a city:-
76 45 67 70

First in the retrain menu, you put ONE of them in - lets go in sequence.
You put unit 76 in retrain.
Exit retrain and mix the 45 onto the 76.
go back to retrain menu and now add the new 41 unit to the retrain menu.
Exit and mix the 67 onto the 41.
Go back to retrain and add the new 28 unit into retrain.
Exit and mix the 70 onto the 28 unit.
Lastly just add the 18 unit to the retrain.

You see the difference next turn.

Be warned about overstrengthing too many unit and entering battle - it can crash if you exceed the amount you are allowed into battle - tho this hasnt happened to me, tho I dont overstrength that much.