There is no fixed size for the flags/banners, print them off in black and white and re-size them to what you think looks best. For 25mm I use any size between 4-7cm long, but its up to you how big or small you want them, as nobody knows what size they really would have been.
@Peter - most of the 28mm collectors are deliberately using bigger than/should/be flags, and that looks good on the tabletop (of course, when you have completed unit
@Maurice, Yep all made by my good self, some bits are copied from photo's online, then coloured over. I totally agree with zzed , about using bigger than/should/be flags, it works, most of the time!!