Jose Manuel Jurado Diaz has a few scripts for us:
Today, we got a new question how much is the size used by a columnstore index table at storage level and memory usage.
TSQL to obtain the total number of rows, size per schema, table and index.
Using the view sys.column_store_row_groups (Transact-SQL) – SQL Server | Microsoft Learn we could see the total number of rows and space usage at storage level.
Click through for that script, as well as a few more to learn how much space and memory that columnstore index is taking.