So my question is: would 20cm in length make much of a difference when it comes to balancing a barbell with 1 arm overhead??
It does, at least for me.
When I joined a gym for my PTTP run this year I tried out side and bent pressing with a bar, because the gym doesn't have KBs. They had three different lenghts. I didn't measure them, but just from guessing the smaller ones were probably 200cm and 180cm or 180cm and 160cm. The big one was a standard 220cm.
The shortest one was definitely the easiest to use.
id prefer the longer bar as it has more room for plates.
Of course I don't know the specifics of the bars you're looking at, but going by my experience with the bars in the gym, even if you use those super big bumper plates you'll at least get 2x 45s and 1x 35 or 25 on either side. Together with the bar that's going to be 25-35lbs you'll still get a total weight of around 280lbs.
Now I don't want to discourage you, but staying realistic, do you see yourself ever bent pressing more than 280lbs?
Hopefully yes
but then you can still get the thin bumpers (e.g. the colored bumpers Rogue uses at the crossfit games).
Why not a standard 220cm bar? It's going to be a bit harder to use for bent presses, but you could use if for all the other lifts aswell.
Granted you can also deadlift or press a smaller bar, but once you decide to do squats or bench presses in the rack you can't use it anymore, because the smaller bars don't fit into the rack.
Another option would be a loadable dumbbell.
Something like this:
Olympic Dumbbell Handle with rotating sleeves - Black Zinc Plated
Used with plates like this:
5kg Fractional Steel Plate chrome
The sleeve lengths are 18cm and the plates are extra thin (1,5cm). You'll get 10 plates (50Kg) on eath side and still have place for a good strong collar (like those 2,5Kg Oly lifting collars).
Together with the dumbbell you'll end up with ~110Kg (242lbs).
You can even get the extra thin 10Kg plates and load up to over 200Kg on the single dumbbell. Any plate that's heavier than 10Kg will have a diameter that's going to interfere with the bent press movement, because they will probably hit your body at some places.