Longest: Lavallee-Smotherman (18 letters)
I originally thought this would be Letourneau-Leblond at 17 letters, but Lavallee-Smotherman reigns supreme here. Both men played with shortened last names on their jerseys though, opting for just the post-hyphenation part.
Forsbacka Karlsson with 17 letters, and none of those pesky hyphens, is the longest to appear in all its glory on the back of a jersey.
All three men also show that long last names do not correlate with long careers as Lavallee-Smotherman had just a cup of coffee in the NHL while the skilled pugilist Letourneau-Leblond and the much shorter initialed "JFK" can best be described as journeymen.
Shortest: Aho, Frk, Ing, Kea, May, Ray, Roy, Roy (3 letters)
There may be some other three-lettered surnames throughout NHL history, but none shorter that I could find. Cliff Pu was drafted by the Sabres but never actually played in the NHL.
Also it may look like I have the same name in there twice, but Roy and Roy are clearly very different names.