There are two possible characteristics of a wave that can be called the wave's "physical size" ... its amplitude and its wavelength.
The amplitude of both Gamma rays and radio waves can be as large or as small as you want it to be, and it doesn't change the name of the category to which we assign those waves. In other words, a Gamma wave is always a Gamma wave no matter what amplitude it has, and a radio wave is always a radio wave no matter what amplitude it has. Either one can have a larger or smaller amplitude than the other one has.
So if we want to look for a physical characteristic that always identifies the type of the wave, we'd have to go with its wavelength. "Gamma" is what we call the shortest known electromagnetic waves, and "radio" is what we call the longest known electromagnetic waves.
So, by definition, Radio waves are always larger than Gamma rays.