What is 1 meter in Unity?

I want to create a copy of the real house in Unity. I have plan of this house and size of the walls in meters.

But how can I know, what is 1 meter in Unity?

I unit is meant to be 1 meter in Unity. You can make the unit represent the length you want but Physics will not follow.

Rigidbody are meant to fall 9.8m/s so a free falling object will travel 9.8 units in one second.

Try this for instance:

    void FixedUpdate()
{
	rigidbody.AddForce(new Vector3(0f,9.8101,0f));
}

and your object will not move. Logic since it is the acceleration of a free falling object.

If you want to change the scale of object you also have to consider all other units to be scale accordingly.

Also, since Unity is a danish company (thanks for that), all kind of weird English measurement are not considered. So no pounds, yards and so on.

If I were you I would just decide that 1 in Unity = 1 meter in real life. As long as the proportions are respected it should be fine.