Further to my earlier comment here is a specific example:
declare @c varchar(50) = '123456789012345678901234'
declare @sql nvarchar(max) = 'select * from xxx
WHERE WEEK in '
IF len(@c) > 24
BEGIN
SET @sql = @sql + '(''81'',''82'',''84'',''83'')'
END
ELSE
BEGIN
SET @sql = @sql + '(''81'',''82'',''84'',''83'',''86'')'
END
EXEC sp_sqlexec @sql