I am new to QGIS and can't figure out how to display a legend for the dot size of a centroid, simple marker. The size of the dot is dependent on a value from a CSV linked to a layer map of Europe. I have two dots per country (different years). This is a snippet of the map:
I was able to create a legend while using graduated symbols. I also tried using graduated and then switch method to "size". However, even in a new project, the only option I get is color:
So this is not an option either. I'm using QGIS 3.8.
Edit: Conerning Josephs answer: I can't find this optione. I do have the advanced dropdown on certain levels, but not on all of them. Here with Screenshots: First level: Second level: Third level: Fourth level: