I just saw this going through the forum main page. I'm not big on Hetalia, but I think I can help with this since I know a bit about flags.
Thats about the average size for a silk used in color guards(Just took out my flag and measured the silk ^^; ). I'd say you'd be best with a six foot pole. If you feel thats too high though, then you could go with a five footer, though the silk of the flag might drag on the floor.
Since its a country flag(I'm assuming so since its Hetalia), I'd go with the six footer. Don't want to disrespect a country's flag.