The most effective method to pronounce length in SQL server

I need to announce the max of length in case condition i.e 7, however, getting mistake syntax near select. Also, To understand more about the length in SQL I also went through some popular resources Here but was unable to understand the concept in a good manner. Can anyone please help?

DECLARE @SQ Int(MAX) 
SELECT @SQ= MAX(LEN(FIRST_NAME))  FROM #table
SELECT @SQ 
-- 7

DROP TABLE #T
SELECT CASE WHEN LEN(SEQ_NUM) = 0 THEN NULL ELSE SEQ_NUM END AS REC_NUM,
CASE WHEN LEN(FIRST_NAME) = 0 THEN NULL ELSE CONVERT(CHAR(SELECT @SQ= MAX(LEN(FIRST_NAME)) 
 FROM #table),RTRIM(UPPER(FIRST_NAME))) END AS FIRST_NAME
 INTO #T
FROM #table

--== Need  7 Char(7) -- Max of len
DROP TABLE #T
SELECT CASE WHEN LEN(SEQ_NUM) = 0 THEN NULL ELSE SEQ_NUM END AS REC_NUM,
CASE WHEN LEN(FIRST_NAME) = 0 THEN NULL ELSE CONVERT(CHAR(7),RTRIM(UPPER(FIRST_NAME))) END AS FIRST_NAME
 INTO #T
FROM #table