We added an awning to our 24 foot Jamboree last year. Its 14 foot but I found that was really the only size we could mount because of doors, windows, storage bins ect. So size will be decided by where you can mount to. Had it done by a company in Oregon. They came and installed the awning and when camping at the coast they came by and installed a window awning for us. You might call them to see what size you can mount. When I called them they knew what would fit. http://www.rvawningrepairs.com/
We spent most of our money traveling... Just wasted the rest..
Really need to look at your Winnie and see where the brackets would mount. You don't want to have a bracket in the middle of a window or in front of a storage compartment door. Also need to see where you have a solid structure to mount to. This will help to determine how big you can go. Once you look at this, it then is a matter of preference.
You will get a lot of opinions on why you want one...seems to be a debate about this. I personally don't have one on my C and wish I did. May get one some day. I did have one on my 5ver and loved it. Even if it is raining I would rather be outside than sitting inside. Also provides a little shade and for me gives me a sense of a larger space.
There will be an awning length that works, and it will depend on exactly which model MinnieWinnie.
Try to find the same model with an awning, to see where the lower awning brackets would mount. There will be vertical bracing and accessory mounting plates at particular locations inside the sandwich wall, rather than regularly spaced framing. You will definitely have a sandwich wall rather than a framed one, Winnebago introduced the technology in the late '60s or early '70s.
You may not need an upper awning track, since Winnebago incorporates an awning rail (and gutter et al) in the extrusion at the top of the wall, which is used for the interlocking joint to the roof. The awning rail in that extrusion has been there for at least the past 15 years, maybe back to 1980's.
A Winnebago dealer has access to the information, even for one that old. The awning used should still be in the parts book. If you can't find a cooperative dealer, an RV awning specialist would know how long, how and where to mount it. But you may not find that expertise at just any RV dealer or particularly big box accessory stores.