What are the length requirement for US aircraft N-Numbers / registrations?
It's not just small aircraft in the US - all aircraft with US registration follow the same rules. The FAA has guidelines on how an N number must be formed. Basically, there are relatively few limits - you can have N + up to 5 characters, up to 2 of which can be letters (at the end).

Here's what the FAA has to say:


  U.S. registration numbers may not exceed five characters in addition to the standard U.S. registration prefix letter N. These characters may be:
  
  
  One to five numbers (N12345)
  One to four numbers followed by one letter (N1234Z)
  One to three numbers followed by two letters (N123AZ)
  
  
  To avoid confusion with the numbers one and zero, the letters I and O are not to be used.
  
  Other Requirements
  
  
  An N-Number may not begin with zero. You must precede the first zero in an N-Number with any number 1 through 9. For example, N01Z is not valid.
  Registration numbers N1 through N99 are strictly reserved for FAA internal use.
  


When an owner registers an aircraft, they can request an N number, provided it's not already registered.