Here is a striking scale comparison of Phobos superimposed upon a medium-size European city, Grenoble, France – does it look unexpectedly small for a Martian moon? Or, true to its name (“Fear”), terrifyingly large and overbearing?
Phobos has a highly irregular shape: roughly 26,8 x 18,4 km (somewhat potato-shaped,and not very large), but it is still 7.2 times more massive than Deimos, the second Mars satellite.
This wonderful photo manipulation was made by French artist Ludovic Celle, who enjoys making nifty concept art of human colonization of Mars: habitats, vehicles and imaginary landscapes. More here The Fearsome Scale of Phobos.