This caution box is telling you to not base your physics simulation on pixels. The pixels your game will be able to show will not be the same across all device, and is not a reliable measure to have your game consistent.
What it suggests is that you use "real world" sized objects in your simulation, and use your rendering engine to display the simulation. You really want to separate the two concerns and think in "real world" objects rather than in "pixels" (these become smaller each year that goes by).
In their examples, they use the concept of OpenGL's viewport, which is used to control what is rendered on the window.
In your case, might want to look into SFML's View. I have not worked with SFML a lot, but this is basically a wrapper of OpenGL's viewport concept.
So for example, you'll simulate your car with real dimensions (say 1.8m x 4.6m), you'll stick a sprite which has that scale to it, but you'll ask SFML to render a parking lot of 200 x 200 units (where units are 'meters' in your game). It'll scale the view appropriately so that it fits in your render window.