I know each team can bring a kind of banner that they put up during the match.
Are their specific rules on how to design that banner?

If you are talking about the Team Standard that will be mounted above the driver station, then yes there are specific requirements. There are several threads on ChiefDelphi that refer to the Standards, and some of them have links to the requirements. Unfortunately I can’t find a link directly on the FIRST site. One would think it would be on the FIRST STRONGHOLD Game & Season Materials page, but no.

EDIT: While I was typing, SGK found the link.

Many teams have flags, banners, etc. in a variety of shapes and sizes. These can be displayed in the pit or stands (don’t block the view of others!) or may be waved by the MC during introductions. These banners do not have any requirements. Just don’t make them so huge or heavy that the MC gets mad at you!