> ...But its length is maximum upto 8,000 charaters
nvarchar(max) can hold over 1,000,000,000 Unicode (double-byte) characters.
If you're having trouble getting all of your data into the variable, be sure you're converting it to the proper type (nvarchar(max)) first.
DECLARE
@blah nvarchar(max)
SELECT @blah = replicate(convert(nvarchar(max), N'0123456789ABCDEF'), 50000)
SELECT len(@blah)
Results:
BlahLen
--------
800000
-Eddie