Given the local soil conditions, how deep should the pole extend into the ground, and what sort of bonding technique would be appropriate? What are the wind conditions throughout the year, and what would the wind drag be on a flag that size? Assume the height of the flag above ground governs the torque exerted on the foundation, so be sure to factor that into the work. Tell us how you'd go about finding a solution.
That is a civil engineering problem, and not a simple one.
It is, if you want to make it difficult, you certainly could.
It could also be a simple 2’x2’ a the top, 3x3 at the bottom, as deep as needed for the soil type. Not difficult at all.
The flagpole will fold, before the footing will give.