What size do you use for varchar(MAX) in your parameter declaration?
For those of us who did not see -1 by Michal Chaniewski, the complete line of code:
cmd.Parameters.Add("@blah",SqlDbType.VarChar,-1).Value = "some large text";
The maximum SqlDbType.VarChar size is 2147483647.
If you would use a generic oledb connection instead of sql, I found here there is also a LongVarChar datatype. Its max size is 2147483647.
cmd.Parameters.Add("@blah", OleDbType.LongVarChar, -1).Value = "very big string";