The usual 3ft wide by 2ft deep should be okay.
Due to your limited rain, I would plant in depressions vs 'hills'.
Plants like 1ft of water per week, so you will have to water that much, mulch to cut down on evaporation.
Add
biochar or really any
carbon to cut down on water needs of the plants.
Seaberry is rat3d for zone 3 - 8 so your zone 5 is okay.
I have some and I have been eating for 2 weeks now.
Also I now have a ton of babies, they also fix nitrogen
Iterations are fine, we don't have to be perfect
My 2nd Location:Florida HardinessZone:10 AHS:10 GDD:8500 Rainfall:2in/mth winter, 8in/mth summer, Soil:Sand pH8 Flat