You need to use the DATALENGTH T-SQL function:
SELECT DATALENGTH(myTextField) AS lengthOfMyTextField
pow·er·cram noun, \ˈpau̇(-ə)r\ˈkram\
Consise, useful information about computers, networks and cloud.
SELECT DATALENGTH(myTextField) AS lengthOfMyTextField
BACKUP DATABASE
AdventureWorks
TO DISK =
'C:\Backups\AdventureWorks.BAK'