About 60 inches to 65 inches would be ideal for most riders. But, for some reason, "one speed" beach cruisers are sometimes sold that are about 75 inches. I would not want to ride up a sand dune at 75 inches. The excuse for hard gearing seems to be that such gears have been used for decades. A poor excuse for designing a bike.
Trek sells a beach cruiser that is set much lower...it feels like it might be around 55 inches. That "easy" gearing is suitable for an "hour a month" rider who wants to ride slowly and easily along a beach, or on a dirt trail. That sort of rider would likely struggle with 75 inches, except on smooth, level pavement.