How to get the size of a varchar[n] field in one SQL statement?

select column_name, data_type, character_maximum_length    
  from information_schema.columns  
 where table_name = 'myTable'

On SQL Server specifically:

SELECT DATALENGTH(Remarks) AS FIELDSIZE FROM mytable

Documentation

For SQL Server (2008 and above):

SELECT COLUMNPROPERTY(OBJECT_ID('mytable'), 'Remarks', 'PRECISION');

COLUMNPROPERTY returns information for a column or parameter (id, column/parameter, property). The PRECISION property returns the length of the data type of the column or parameter.

COLUMNPROPERTY documentation