It is [currently fixed to 16px width](https://github.com/geany/geany/blob/1f71ccd40ac4e6754d7609bec7f81773fa181146...), but I guess a setting could be added by someone to fix this.
OTOH it could possibly be interesting to have it dynamically follow the line height by default, so it would feel natural -- and the same could apply to the fold margin. That'd be a little more work, and would probably need to check if it actually works well, but it seems like a good idea from the top of my head.