In essence your flags are no different to a printed ad, a flyer, or a Google or Facebook ad.
You want to attract someone’s attention and for them to take action. All the thought, creativity, and testing you might put into your ad is also required in the design of your feather flag.
Some exhibitors will choose the tallest, the highest, with their flags strapped to the top of their stand or marquee so their branding can be seen from afar. Others may choose something more modest, sited at eye level to attract passing traffic. Both options can work well for you, but only if all the design elements, the text, layout and colors, work in unison, projecting your branding to best effect.