Sorry about my mistake. I should not trust document from Microsoft 100%. Here is a sentence from "Tabular Data Stream Protocol Specification".

"A type with unlimited max size, known as varchar(max), varbinary(max), nvarchar(max), which has a max size of 0xFFFF,..."

You can get the specification from http://download.microsoft.com/download/a/e/6/ae6e4142-aa58-45c6-8dcf-a657e5900cd3/%5BMS-TDS%5D.pdf (page 36)

By the way, Our product communicate with SQL Server using TDS protocol, we do not really care about how long varchar(max) and nvarchar(max) columns can hold.