Calculate Avg BLOB size

select *, gigabytes / 1024.0 as terabytes
from (
  select *, megabytes / 1024.0 as gigabytes
  from (
    select *, kilobytes / 1024.0 as megabytes
    from (
      select *, bytes / 1024.0 as kilobytes
      from (
        select avg(datalength([Value])) as bytes
        from <schema>.<yourtable>       
      ) a
    ) b  
  ) c
) d