The fact it measures 20x20m wasn't an issue, but apparently it's against regulation to mention the opposition...
The Sex Pistols-inspired design was paid for with donations from supporters via the 'My Old Man Said' fan group, who have released this statement: “While there’s a slight edge of humour there, it’s not disrespectful in the slightest to Arsenal fans, as the intention behind the flag was to announce to the world with some bravado that Villa are back.”
“We have asked the FA for the rules and a risk assessment of this flag going to Wembley, but no such evidence has been presented to us.”
A petition calling for the flag to be allowed in to Wembley Stadium this Saturday has so far gathered 4,795 signatures.