However, it seems the size of the logo is capped to a maximum size.

That is correct - the logo file is always loaded at its native resolution, and then scaled according to the 'logo-size' setting. However that size doesn't relate to the image size of the logo, it is expressed as a percentage relative to the screen width. That may seem odd, but keep in mind that the extension serves a very specific purpose, namely: having a Fedora-branded default wallpaper without restricting distribution of the wallpaper due to trademark law. So to achieve the effect of a trademarked wallpaper, the logo needs to be scaled with the background, not stick with the image file size.

So no, we won't use the original image size, sorry.

You may argue that the current maximum size - 15% of the monitor width - is too restrictive and should be increased, but 20-25% is likely still too small for your use case, while (IMHO) overly excessive for a logo.

To be honest, I think rather than turning your agenda into an SVG and feeding it into the background-logo extension, your use case would be much better served by a dedicated extension that tracks your agenda source (calendar events, todo, .txt or whatever) and overlays it over the background.