SQL Functions

Returns the number of characters of the specified string expression, excluding trailing spaces.

To return the number of bytes used to represent an expression, use the DATALENGTH function.

The following example selects the number of characters and the data in FirstName column

SELECT LEN(FirstName) AS Length, FirstName
FROM Sales.vIndividualCustomer  
WHERE CountryRegionName = 'Australia';

Source: LEN (Transact-SQL) - SQL Server | Microsoft Docs

This function returns the number of bytes used to represent any expression.

To return the number of characters in a string expression, use the LEN function.

This example finds the length of the Name column in the Product table:

SELECT length = DATALENGTH(EnglishProductName), EnglishProductName  
FROM dbo.DimProduct  
ORDER BY EnglishProductName;  

Source: DATALENGTH (Transact-SQL) - SQL Server | Microsoft Docs