There are several ways to find the size of SQL Server databases. However, the easiest method to get the list of all the databases along with their sizes is by using the system stored procedure sp_databases. Here is an example on how to execute this stored procedure and the result.
EXEC sp_databases; /* Result */ DATABASE_NAME DATABASE_SIZE REMARKS DnnDev.me 41280 NULL master 6592 NULL model 16384 NULL msdb 42368 NULL MtbToolsDB 291840 NULL SirApp 16384 NULL SirDB_201804 167552 NULL tempdb 16384 NULL Test.MyTecBits.com 24576 NULL WideWorldImporters 3379200 NULL
NOTE: In this, the size of the database is given in kilobytes. Also, this method will not give you the sizes of log file and data file separately. So, you can use this method to find the database with large size.

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Reference
- More about sp_databases at Microsoft Docs.