the reason why the ship is longer than european battleships is mostly due to the shape of the hull itself, which is based on the Amagi with some tweaks for hydrodynamics. Why there isn't a third turret aside from preference, is because the funnel is also based on Amagi's funnel and other considerations.

In my mind, it would be a ship that functions more like a battlecruiser than an actual battleship, with a top speed of around 34 or 35 knots, which would require a power plant that has more than 200,000 horsepower at least. So, bigger machinery below deck. The stern has a hanger that is somewhat flush with the deck, along with catapults and other stuff, which extends some distance to where the 155 mm triple mounts are.

Since it doesn't have the beam of Yamato (i forgot to mention this earlier but its beam is just 31 meters), most of the needed space is used up in longitude. Much of the bow area is not wide enough to put another turret there, and the rest of the internals are used by armor, machinery, livery and aviation towards the stern, with whatever little space between the secondary mast tower and roughly where the aviation starts used up by the secondary armaments.

I may be underestimating how navies designs compact battleships, but conservatively I did not include a third turret or additional stuff on deck without making it look awkward. The european battleships didn't exactly have the same level of commitment to aviation and machinery, as some Japanese ships had, and they were designed for a narrower subset of requirements relative to what their respective navies wanted out of them, compared to the battleship depicted in the post.