On vintage trailers, another alternative to an expensive ZipDee is a rope & pole awning. They are much less expensive and keep in character with the old-time vintage feel. There are two readily available options - Vintage Trailer Supply
& Marti's Awnings
Either way, the awning typically goes the full width of the flat panel. There should be an awning rail on your trailer already - it's the little c-shaped channel that you feed a rope or rod trough. If you are going the rope & pole route, just measure the length of that. If going with a ZipDee, I would have the dealer you order it from measure it for you.