It sounds like the same product that we do a lot of here. There's no technical reasons for a particular margin, as they can be full bleed (print right to the edge), so it's a subjective design decision.Take into consideration the content and the context. It could be a something with little content, intended to draw people from a distance (larger margins), or it could be something with a lot of content that will be read from a short distance (smaller margins).
I tend to do these as 1:1 scale, but bear in mind that InDesign's Optical Kerning will be too tight when you do this.
Very helpful, thank you.
Just to expand on this, if preparing artwork for banner stands such as retractable pull-up banners, be aware that there is a portion of the banner that is hidden through the spring-loaded mechanism at the base of the stand. However, the providers making assembling and printing the stands usually have a specification sheet on how to prepare the banners - ask them in the first instance for the specification sheet.