Open turret does not have a significant effect on vehicles silhouette. Historically it also proved not effective and m36 where actually refitted with folded armored roof after WWII.
But all that is rather irrelevant in game. Even with target size of 24 a PzIV has 60% chance of scoring a natural hit at range 40 or a 30% is moving. There is really no need to lower that chance.
Puma is a reconnaissance vehicle comparing it with a TD is rather misleading especially since the stug has the normal SR of 35.
In the end of the day historic accuracy is a good thing and there should be an effort to be achieved but not if that effect balance in a negative way.
1)
when people say "open top" turret they don't just mean the lid is open. They mean the turret is missing the top ~20%.
For example, the m10 Achilles (which use the same hull and turret as the m10 wolverine) is listed as 2.57m on wiki. The vehicle achieve this height partly through its use of an open top turret.
The in game "size" stats is clearly a vague representation of the vehicle's size IRL.
As some clear example, the size 17 jp4 was only 1.85 m tall. The size 20 stug was 2.15 m. the panzer4 is 22. Larger vehicle get bigger size.
The height listed on wiki for the jackson is 3.28 m over the .50cal, which is clearly mounted on a elevated gun mount. The best stats I could find of the jackson's height is 2.71m, similar to the panzer's 2.68m on wiki.
the panther tank is 2.99m IRL, and 24 size in game.
2) the puma have a 50m range gun and a 50m sight. It can spot for itself and still out-range most enemy.
only looking at the puma as a "recon" unit is demonstrating a lack of imagination.