The player can not.
The player can only estimate the size of the objects on the screen by comparing them to other objects on the screen. When the player does not have a reference object in which he can estimate the size from his experience in the real world, then they can only guess at what scale the world is. They could be playing a character the size of a flea or the size of a giant. They will not be able to say it.
A good experiment in this regard is the "rat map" genre, a popular theme for maps made by players in Counter Strike and other first person shooter games of the late 90s and early 2000s. In this mapping issue, players feel as if they are reduced to the size of rats while playing in a regular scale room. But in reality the character-players are still the regular size. What has expanded is the world that surrounds them. However, the illusion works quite well. Why? Because players recognize many objects they know from real life and see that they are approximately 20 times larger than they should be. So their brains tell them "with that many objects are 20 times larger than normal, the most logical explanation is that you and the other characters are reduced to 1/20 of their normal size". which is actually unlikely due to how gravity works and the law of the square / cube, but our primitive monkey brains do not realize that 🙂