how to change the mesh of the human character by scripting in unity 4 ?

Hai everybody...... iam trying to change the mesh of the human character while running of the game. for example my player is in running state if player takes any special power ,player size want to increase or decrease........i want some sample codings or projects or links for how to change the mesh of the player dynamically......is this possible to change the players mesh according to my point of view....digital tutors are more better.........Thanks in advance......