The command line editing routine limits the command input to 64K characters, mostly because (1) you don't need anything bigger, and (2) it really, really slows down the editing when you get to this size, particularly when inserting characters, and (3) the cursor positioning on a command that spans many screen pages is nasty, and (4) there's a couple of Windows APIs that gork on the cursor positioning when the size gets larger than this.
But this doesn't affect how large the command line can grow to be after alias & variable expansion, or the size of the command line read from a file. (But Windows will likely explode with the really large command lines anyway.)