Notepad++ won't open a file without newlines if it's larger than 32K. This isn't because of file size; N++ happily opens files > 100M. I often need to view a 500K file with no newlines. I'd certainly encourage the N++ developers to increase the maximum linelength. Sure, it's ridiculous to attempt to manipulate a 500K "line" in a 100 character window, but it's better than nothing.
It's a known bug, with which I don't have the solution yet.