hi! first, a plug for /r/vexillology, the flag sub, where you might find more information. not discouraging more answers here, but you might be interested in some previous related threads:

origins:

• Where did the concept of states possessing a symbol (flag, coat of arms, crest) come from?

• How did flags as we know them come to be?

shape:

• Why is the rectangular flag the standard? When did it become so? Why is it only Switzerland and Nepal that break this trend?

• Why are flags across the world the size and shape they are?

• Why are so many flags the same basic size and shape? Is this mostly a modern 'standard'? (excuse the pun)

colour & pattern: there have been dozens of threads in this sub asking about colours (why stripes, why 3 stripes, why certain colours, why the white flag, why stars, why writing...); a few of the more common themes are included in the FAQ:

• Why do flags have similar colours?