In ArcGIS Pro compaction or compression of geodatabases are run through the geo-processing tools. In ArcGIS Pro you cannot evaluate the performance of these tools, i.e. you can't review the reduction in database size as you can in ArcCatalog as you can add a size column (another good reason to ensure ArcCatalog is not retired?).
I have found no way of displaying the geodatabase size in ArcGIS Pro.
It would be good and a fairly simple improvement to the existing geo-processing tools to report a before and after geodatabase size. Currently the tools only report if they have succeeded or failed.