SQL Server Determine Physical Size of Table Columns

You should be able to use the datalength function, something like

select sum(datalength(yourfield))
from yourtable

This will summate the datalengths of all the entries of that field in the table - it will not account for overheads such as variable length field pointers, space in the nullability bitmap etc.

Select SUM(DATALENGTH(columnsA)) / 1024.0 AS KB

For view size of "all columns a table", you can first generate fields of your table with:

SELECT 'SUM(DATALENGTH('+Column_name+')) / (1024*1024) as '+Column_name+'_MB,'
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = '--TABLE_NAME--'

and next, run this query:

SELECT
    /*result previous query (with remove last comma)*/
FROM --TABLE_NAME--

This will return all columns in a specific database with their data size and can easily be updated to only return the numbers for a specific table or column.

USE [YourDatabase]
IF OBJECT_ID('tempdb..#temp') IS NOT NULL DROP TABLE #temp

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
CREATE TABLE #temp (tablename varchar(max), columnname varchar(max), sizeinkb float)

DECLARE MY_CURSOR Cursor LOCAL FAST_FORWARD 
FOR SELECT table_name, column_name, table_schema FROM INFORMATION_SCHEMA.COLUMNS

Open My_Cursor 
DECLARE @table varchar(max), @column varchar(max), @schema varchar(max)
Fetch NEXT FROM MY_Cursor INTO @table, @column, @schema
While (@@FETCH_STATUS <> -1)
BEGIN
    DECLARE @sql varchar(1000) = 'INSERT #temp SELECT ''' + @schema + '.' + @table + ''', ''' + @column + ''', sum(isnull(datalength([' + @column + ']), 0)) / 1024.0 FROM [' + @schema + '].[' + @table + '] (NOLOCK)'
    EXEC (@sql)

    FETCH NEXT FROM MY_CURSOR INTO @table, @column, @schema
END
CLOSE MY_CURSOR
DEALLOCATE MY_CURSOR
GO

SELECT *, sizeinkb / 1024.0 sizeinmb FROM #temp ORDER BY 3 DESC