Upfornt 2 object length is a minimum. Downflow 5 is a minimum. Better is to have more distance upwards and downwards. You can keep the top boundary as symmetry if it is far away from the free surface. I would say more than 0.5 of wave length if you are modeling open channel waves better to have more. Using pressure outlet at top boundary not really far away will result in affecting the overall free surface behavior. Same with symmetry which is actually wrong if you keep it near the free surface.