Performance Of NVARCHAR(n) For Large Strings

What are the performance or storage implications of using a large value for NVARCHAR? For example, if I specify a NVARCHAR(4000) column just to cope with the rare case there are strings that long, but have a table full of strings 255 characters long, is the performance identical to specifying an NVARCHAR(255) column? Is there a reason then NOT to specify NVARCHAR(4000) on everything? i.e. does the query optimizer use it?



I know how long a row is and how many rows can fit in a page (4096 bytes) affects performance, but my understanding is that NVARCHAR only stores the characters needed so it wouldn't be affected unless there was actually a longer string. I wanted to know if there are any other considerations to using a large value here.



Also, how does NTEXT compare to using NVARCHAR? I noticed the documentation said it used a new page after 256 characters, which sounds like it's different from how a 4000 byte NVARCHAR would be stored?