erm 11", lol, i use a 16" barrel cos it seems to balance the marker a bit better not noticed any power increase but a good barrel will be more accurate, never tried a 20" barrel tho, maybe a bit much, cant go wrong with a 12,14,or 16" barrel. (in my humble opinion) hope it helps
If you chrono your marker at 300fps with a 12" barrel then move to an identical bore size barrel which is 16" long, your velocity will drop. So a longer barrel will require more air to maintain the same velocity as a shorter barrel.
The main thing that impacts accuracy is the paint to barrel match rather than the length however I think most people agree that the sweet spot for barrel length is 12" or 14". Shorter barrels do seem to impact accuracy, while there's no noticeable benefit from going to longer lengths.
loads of things change how well a barrel works, effective barrel length, porting, bore match. basicly anything over (in my humble opinion) 14/16 inch is too much and pointless, you basicly want around 10/12 inch effective length depending on range your looking at.