What are the maximum number of characters that can preceed the .fi
extension?  For example is ab.fi acceptable, or is abc.fi the shortest
domain of the .fi extension allowable?

Dear binhaveb,

As the Finnish Communications Regulatory Authority states on their
official website, the shortest possible .fi-domain must have the form
"abc.fi". The longest possible domain may have 60 characters:

"WHAT KIND OF CHARACTERS CAN YOU USE IN YOUR DOMAIN NAME?
-the letters a - z 
-the numbers 0 – 9 
-a hyphen 
-the name can consist of 3 - 60 characters"

Source:

FIOCRA (Finnish Communications Regulatory Authority): An ABC of
Applying for Domain Names
http://www.ficora.fi/englanti/internet/abc.htm

Hope this answers your question!
Best regards,
Scriptor

Thank you.  Fast and complete.