'Highball' was similar in shape to the original design for 'Upkeep', being semi-spherical.
I've checked on the dimensions of 'Upkeep', and although the original design, with the spherical outer casing, was 7 ft 6 ins in diameter, the final weapon, just the cylinder, was 50 inches in diameter.
From memory, the placard at Newark states that it is an actual 'Upkeep', and it's also included in the listings of surviving examples, and I guess my estimate of 40 inches wasn't far out.
I think the angle of the photo probably makes it look smaller than it actually is, and I do remember it looking larger 'in real life', when standing nar it at the museum.