I'm learning to create panoramic photos. I'd like to know if there's any recommended proportion to determine its size. I have the feeling that 4:1 is fine, but I could'nt find any pattern.
There is no set rule for this; it all depends what you want to photograph. If a photo is wider than a 4:1 ratio, it will look a bit too thin, but you could shoot a whole 360° panorama and make an interactive QuickTime panorama that lets you pan and zoom within a window.
If the end result is a good photo, it shouldn't matter what the exact proportions are.
A lot of the recommended sizes for different image types (2:3, 4:5) are based on the availability/cost of printing in those sizes. For instance, 4:5 is only common because many portraits are printed as 8"x10".
For panoramas, there hasn't been any agreed upon standard, and there are not a lot of printers that handle panoramas outside of a custom job.
I chose a rather odd-sounding ratio of 2:9 (or 1:4.5 if you prefer) for the panoramic printing option of my site (OddPrints). This is the aspect ratio of panoramic photos taken with iPhones.
I agree there should be more "standards" but hopefully if the iPhone panoramic feature prove to be popular, this standard size may emerge. Hopefully, manufacturers of picture frames and mounts will read this and start making affordable frames at these sizes ;)
There is no set standard or "best practice" ratio for panoramas--it all depends on the subject, the type of panorama, and how you're presenting it.
If you're presenting the panorama interactively, then you generally just need to worry about the presentation window aspect ratio, and most would go 16:9 to fit the most common screen aspect ratio out there today, and you're mostly worried about whether the resolution of your panorama is high enough to create a good viewing experience, since only a smaller portion of the image is seen at a time.
With cylindrical panoramas, you'll mostly be worried about vertical coverage to avoid looking like panning through a thin strip. If you plan to present a 360ºx180º panorama in equirectangular format, then it must be 2:1 by the nature of the mapping.
If you're planning on printing, then available paper and frame sizes might determine a more favorable aspect ratio over others, just from a cost perspective. But there's also nothing that says you can't shoot and stitch multiple rows of images to create a panorama, or that you have to hold the camera in landscape orientation when shooting one. It doesn't have to be a narrow strip to be a stitched panorama.