The rule of thumb is the length of the flag should be equal to 1/4-1/3 the length of your flagpole.  For instance, if you have a 25 foot flagpole, you could buy either a 4’x6′ flag or a 5’x8′ flag.  It’s important not to overload your flagpole with a ton of extra weight.  This will have an affect on the amount of wind a flagpole is able to take.  It’s also important not to have too small of a flag on the flagpole as this will cause the flag to look tiny compared to the pole it’s on and the visual image of the flag will look different.  Stick with the 1/4 to 1/3 rule and you should be fine.