A concise recommendation for flag design is to imagine it reduced to the size of a postage stamp, if it cannot be successfully reduced without losing detail then it is a bad design. There are several guides to flag design which outline the broad, tried and proven principles, that create effective flags.
Also available are the more general
Vexillographer (flag designer) Philippe Bondurand identifies ‘seven rules’ to consider in flag design, at his website and
further guidance is offered by fellow vexillographer Dave Martucci at his page.