What is the size of a image field content in SQL Server?

SELECT DATALENGTH(imagecol) FROM  table

See MSDN


Different display styles:

SELECT DATALENGTH(imagecol) as imgBytes,
       DATALENGTH(imagecol) / 1024 as imgKbRounded,
       DATALENGTH(imagecol) / 1024.0 as imgKb,      
       DATALENGTH(imagecol) / 1024 / 1024 as imgMbRounded,
       DATALENGTH(imagecol) / 1024.0 / 1024.0 as imgMb
FROM   table

Example output:

imgBytes    imgKbRounded    imgKb       imgMbRounded    imgMb
68514       66              66.908203   0               0.065340041992