I'm surprised that no one has mentioned the Guide Specifications for Design of Metal Flagpoles, ANSI/NAAMM FP 1001-97.
You didn't state the flag material, but the code provides values for nylon and cotton flags, and for polyester flags.
For nylon and cotton: Wf = .0010 x V^2 x Af^.5 x Ch
For polyester: Wf = .0014 x V^2 x Af^.5 x Ch
where V = design wind velocity in mph.
Af = flag area in ft^2
Ch = 2.01 x (z/900)^(2/9.5) where z = height of the
pole in feet.
It's probably a good idea to obtain this code, or if you have the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminares and Traffic Signals, 1994, there are mostly the same procedures except the AASHTO Standard doesn't have the flag load formulas.