Prova detta:
Select schema_name(T.schema_id) As SchemaName,
T.Name As TableName,
Sum(C.max_length) As RowSize
From sys.tables T
Inner Join sys.columns C
ON T.object_id = C.Object_ID
INNER JOIN sys.types S
On C.system_type_id = S.system_type_Id
Group By schema_name(T.schema_id),
T.Name
Order By schema_name(T.schema_id),
T.Name