For this release we improved how the legend for QGIS servers handles the size of symbols which are scaled by map units. Whenever possible, the server will determine the legend size based on the map canvas and resolution. Even more, for cases where this information is not available in the GetLegend request, it is now possible to configure a default scale in the project, symbols in the legend will then be drawn in the size they will have when rendered at this scale.
This feature was developed by David Signer, OPENGIS.ch