A good general rule:  The Length of your Flag should be at least 1/4 the height of your flagpole.  For example, a 12’ Flagpole would require a 2’ x 3’ flag and a 20’ flagpole a 3’ x 5’ flag.  Most “porch” flagpoles are 5’ or 6’ and made for a 2’ x 3’ or 3’ x 5’ flag .