You can do so using oql. if the attribute of type Enumeration is called MySize the OQL will be
SELECT COUNT(*) as _Sum, MySize
FROM YourModulename.MyTable
GROUP BY MySize
Try out this OQL by clicking in the ProjectExplorer→Add Other→Dataset. Sourcetype OQL.
To call this OQL from a microflow import the AppStoreApp “OQL". See this shared microflow.
The result will be
3, Low;
5, Medium;
7, High;