[Solved] SQL Field Length

You need just this:

SELECT @list = seg_tag 
             + '01' + RIGHT('0' + CAST(LEN(ID_Type) AS varchar), 2) 
             + ID_Type  
             + '02' + RIGHT('0' + CAST(LEN(IDNumber) AS varchar), 2)
             + IDNumber
FROM #TEMP_TABLE_ID

Or

SELECT @list = seg_tag 
             + '01' 
             + CASE WHEN LEN(ID_Type) < 10 THEN '0' ELSE '' END + 
             + CAST(LEN(ID_Type) AS varchar)
             + ID_Type  
             + '02'
             + CASE WHEN LEN(IDNumber) < 10 THEN '0' ELSE '' END + 
             + CAST(LEN(IDNumber) AS varchar)
             + IDNumber
FROM #TEMP_TABLE_ID