They already do 140mm fans, the F14. They be named by their measurement in cm. Not sure of any F12 that is 140mm. It would be a F14.http://www.arctic.ac/en/p/cooling/case-fans/77/arctic-f.html
F12: 120 L x 120 W x 25 H mm
F14: 140 L x 140 W x 25 H mm
F12 : 74 CFM / 125.7 m3/h
F14 : 77.3 CFM / 131.4 m3/h
F12: 0.3 Sone
F14: 0.5 Sone
F12: 107 g
F14: 146 g
F12: 120 mm, 600 - 1,350 RPM
F14: 140 mm, 550 - 1,350 RPM
All the F12 and F14's are the same (in the above info), some come with slightly different add-on attachments, but the core stuff above is identical.
It's about double the price going from F12 to the F14. So you gain a whooping (sarcasm) 2.7 CFM with an increase of 0.2 Sone. Which works out to be approximately 2db. It's still consider quiet since that is about 24 - 25db. Which is close to background noise is most peoples home.
It appears they tried to retain as much of the design and parts from the F12 and just made it bigger without properly adjusting the parts.
As you no longer see any useful increase going up to 140mm, they should of used a bigger motor or a lighter/better fan blade design.
In short the F12 is a good deal. Don't bother with the F14.