For the pixels, you look at the sprite textures you made, measure the height of a human sprite in pixels, depending on the type of person depicted you divide it by a number between 0.3 and 2.4, and then you know how many pixels is a meter.
I did not use Box2D, but you should be able to convert the vectors easily by assigning the components from a SFML Vector2 to a Box2D vector and maybe applying a scale factor and do same the other way around, too.
