Or you might prefer this restatement of the "constant light speed" postulate of relativity: In terms of the meter (the SI unit, based on the above chosen value for c), any physically measured sizes are independent of the motion of the observer, assuming the observer is in an inertial frame. For example, the volume of a cube containing one kg of water will be 0.lm on a side in any inertial frame.
In the past one would ask for the value of c in terms on one of the old definitions of the meter (the wavelength of a particular light wave or the distance between two marks on a metal bar); now one would ask the opposite question: Does the wavelength of a particular light wave change if an inertial frame moves? Do the two marks on a bar change their separation in a moving frame? But be careful: The question has to do with experiments made by an observer moving at the same velocity as the object being observed. The well-known relativistic contraction of length deals with an object in motion with respect to the observer.
Also, you will note that the SI unit of meter depends on the speed of "light." One could well check whether red light, green light, radio waves, gamma rays, left-circularly polarized light, etc., all travel at the same speed. (They do, though I don't know how accurately these observations have been made.) Experiments are currently under way to check if the neutrino has a rest mass (otherwise, it too travels at c); maybe in the future one can check whether gravitational waves travel at c.