You have to define the metre your self : there is no definition which I am aware of...
For example, if you have a sprite of a ten year old which is 64x64 pixels, then you can say that 1 metre is 50 pixels. This will mean that your sprite is 1.28m tall.
To implement this, you have to scale your box2d measurements up by 50 and your sfml measurements down by 50. Alternatively, you could set up a view, like this:
sf::View gameView ( sf::FloatRect (0, 0, 800 / 50, 600 / 50) );
Hope that helped.
