The player can't.
The player can only estimate the size of objects on the screen by comparing them to other objects on the screen. When the player has no reference object where they can estimate the size from their real-world experience, then they can only guess what scale the world is in. They could be playing a character with the size of a flea or with the size of a giant. They won't be able to tell.
A good experiment in this regard is the genre of "rats maps", a popular theme for player-made maps in Counter Strike and other late-90s/early-2000s era first person shooters. In this mapping theme, the players feel as if they were shrunk to the size of rats while they play in a regular-scaled room. But actually the player-characters are still the regular size. What's scaled up is the world around them. Nevertheless, the illusion works pretty well. Why? Because the players recognize a lot of objects they know from real-life and see that they are about 20 times as large as they should be. So their brains tell them "with that many objects being 20 times larger than usual, the most logical explanation is that you and the other characters are shrunk to 1/20th of your normal size". which is actually unlikely because of how gravity works and because of the square/cube law, but our primitive monkey brains don't realize that :)