Auto Completion, Desired Word Length

[Artist]

Hi,
 
 I guess this would be an interesting exercise.
 How I can make auto completion M-/ working for desired number of
length.
 Example: If I would like to complete 'st' and my length is 5, it
should  show the words, still,style etc.. Also after I auto-complete
the word, I would like to enter the word in the buffer. Then on the
next M-/ it should pickup where it has left before.

Thanks,
Artist.