I don't think there is really a standard scale for what 1 unit in 3D space actually represents. This may not be true when using physics but I suspect that that can be set up to mean whatever you want it to mean.
What I tend to do is fit whatever I need within the standard camera minimum and maximum camera range. The maximum default camera range is 1000 and the minimum is 1 (I think).
So, if I was to make a soccer game, I would want the player to see the entire pitch so the maximum I would make the stadium is would be 1000 units from one corner to the diagonally opposite corner. I would then make the soccer ball in scale with the pitch, whatever that happens to be.
However, if was to make a room escape style game then the a thousand unit might be then length of a single room that's no more than, say, 5 meters long and then size all the furniture according to that.
I've never changed the default camera range as I think you can get problems with the z depth if you make it too big.
The other issue is minimum camera range as if you allow the camera to get to close to a wall then the wall may be culled, allowing the player to see through it. I find this is more important in games where the camera collides with walls (such as in an first person game).
The other thing to consider is that generally for first person games, the maps tend not to follow 1:1 scale. I read somewhere that rooms tend to be three times bigger than in real life (furniture will be to scale) allowing more space between objects so the player's not always crashing into things.
You'll need to play about with things until it looks/feels right. For instance, something as simple as the height of the camera above the ground can have an huge influence on the feeling of speed.
Just to finish, I'm currently making a third person arcade/action shoot-em-up/beat-em-up and then main character is five units high and the corridors are about 20 units high.