How to find a biggest document size stored as blob in oracle table?

I am able to get the overall size of the Blob by using below query

SELECT SUM(DBMS_LOB.GetLength("col_name"))/1024/1024 AS SizeMB FROM table_name;

However, I need to find the size of the biggest document stored in the blob.

Note : We are using 19.18 Oracle Enterprise edition standalone database.