Yeah establishing scaling in video games is a bit of a problem as humans have the ability of a 190 degree peripheral vision in relationship to a 2d plane limited to 90-100 degrees of FOV.
Important fact as to make things look “good” as compared to real world measurements you have to trust what your eyes are telling you and not so much the math that says it’s right or wrong to compensate for effect.
A good starting point would be to research movie making production values as they have the same problems as to scale as video games do. Next time you watch a movie with guns and the guns look bigger is because they are bigger so they “read” well even on the big screen.
As for a 3d environment there is no such thing as a true measurement or scale except for the unit which is not a measurement but a means to establish a measurement relative to world space. A unit can be anything from an inch to a mile and in the case of UE4 a unit is a centimeter. It’s an important value to consider when using physic based lighting or simulations and after that everything else is an illusion of what your eyes are telling you is true “relative” to everything around it.
See the trend here?
Using the player model as a reference point could be a starting point but you would have to consider relevance as to scaling for effect with in context of the result your looking for and as to how it will be read from the players perspective.
The pictures you posted, for the kind of effect your going for assuming foreboding, real world scale would work well from the third person perspective but maybe not so good in the 1st unless going for the walls are closing in effect but won’t work at all if the game was a FPS. Continue using real world scale in the rest of the environment usually the result everything will “read” smaller.
Granted this is all observation and not so much fact but the long way around in saying let your eyes tell you what looks good and not the math but in my experience going 10-15 % bigger in scale relative to real would measurements is better than being to small giving that playing in a box effect.
You can set your games default FOV to 190 degrees.