title_body: Determing the frame's hole mount length, for the right mount size motors How do you find out the frame's motor mount mm length? 
  
  And what motors are generally for 3 inch frames and 5 inch frames? 
  
  Example: 
  3inch frames are generally for 1206 or 11XX motors?
  
  5inch frame are generally for 2306 or 22xx motors? Does 23xx or 22xx motor number stand for an identifier?
upvoted_answer: This blog post by Oscar Liang has a table identifying the type of motor that is used for each frame on 4S (other voltages would only affect the KV). Here is a screenshot of it:
  
  
  And yes, 22XX means any motor with a stator diameter of 22mm and with any height, as it is the stator diameter that will determine the mounting pattern.