In QGIS 3.10.2 I have a map that I want to show points by colour using one category and size by another. I have tried to follow the instructions here: https://docs.qgis.org/3.10/en/docs/user_manual/working_with_vector/vector_properties.html#proportional-symbol-and-multivariate-analysis. This I have done but a consequence is that in the Layout composer the legend symbol is either missing or very tiny.
This is my map with just a colour classification:
I then add the instructions to re-size by a second attribute (I converted a 4 category variable into the numbers 1, 2, 3 and 4 and called it SizePoint)
Now the legend in the Layout composer does not show a dot at all
Now here this isn't a problem because all the dots are of the same INSTANCE. But there are other maps where there are more than one instance:
You will see that there is no indication in the legend what INSTANCE 1 or INSTANCE 2 are.
I did try and look at the very specific help text here:
But I do not know what the classification frame is or see a Change button.