QGIS3 data defined overide not active

Question

In QGIS 3.4 MADEIRA, I get no active "data defined override"; I click on the sybol right toio "Size", that I want to modify with a function. The words "data defined override" are pale grey and italic, and when I click on "description " I get this

I suppose I must activate it somewhere, but I was unsuccessfull to find where.

Answer 1

"Data defined override" is more of a caption than anything else. You either have to choose a field you want to derive your symbology size from or enter an expression.