I would go with placing the Arc along the shorter wall. My living room has very similar dimensions and my Arc/Sub/Play:1s setup sounds great with the Arc on the shorter wall. Less distance for the side firing drivers to reflect sound off the wall will produce better virtual surround on the sides.

But a more important factor is where your primary seating location will be. Ideally you want to be seated 5-7 feet from the Arc to hear the upward firing drivers when watching Dolby Atmos content.

I’d worry more about optimal seating for viewing than the placement for sound.

Once you have the TV, Arc, Sub and your seating in place you need to position the surrounds. Again go with your ears for what sounds best to you.

TruePlay every setup before testing.

Our TV room is horrible. No option on TV and Arc placement, same for the surrounds. All are in highly “not recommended” positions but after TruePlay the system sounds pretty good. Weakest point is ATMOS with the odd angled ceiling orientation, still pretty decent.

Generally, I prefer placing the TV on the short wall, but as Stanly_4 suggests you want the speakers to be placed symmetrically with respect to the listener and the surround speakers should not be very close to a listener because one surround will dominate for that listener.

Another consideration is the location of light sources relative to the TV screen. A strong light source directly opposite the TV is distracting. (because the TV screen tends to behave as a mirror)