Is one of these what you are looking for?-- THIS?
SELECT model_size, model_type, actual_qty, process_qty, order_type,
CASE WHEN order_type = 'Some X' THEN model_size END AS ModelSizeForSomeX
FROM TheTable;
-- Or this?
SELECT model_size, model_type, actual_qty, process_qty, order_type,
FROM TheTable
WHERE order_type = 'Some X'