radial-gradient(75px circle  at 50% 50%, red, red 50%, white 74%, grey 76%, grey 79%, rgba(80,80,80,1) 81%, grey 83%, grey 86%, white 89%, transparent, transparent 98%, black 98%, black 99%, black)

Size can be set explicitly as in the above example or it can be set implicitly by telling it roughly where to put is and how to render it.

In this example a simple radial gradient is generated with the outer black portion beginning at 100% and going on forever. There are four modes of radial gradient innate sizing, they're all determined by where the theoretical circle is that can be drawn without going outside the four things: the closest side, the closest corner, the farthest corner or the farthest side. Knowing the black part starts at 100% there four are perhaps easier to understand with how they differ.

closest-corner

radial-gradient(closest-corner circle at 85% 15%, red, red 50%, white 74%, grey 76%, grey 79%, rgba(80,80,80,1) 81%, grey 83%, grey 86%, white 89%, transparent, transparent 98%, black 98%, black 99%, black

closest-side

radial-gradient(closest-side circle at 85% 15%, red, red 50%, white 74%, grey 76%, grey 79%, rgba(80,80,80,1) 81%, grey 83%, grey 86%, white 89%, transparent, transparent 98%, black 98%, black 99%, black

farthest-corner

radial-gradient(farthest-corner circle at 85% 15%, red, red 50%, white 74%, grey 76%, grey 79%, rgba(80,80,80,1) 81%, grey 83%, grey 86%, white 89%, transparent, transparent 98%, black 98%, black 99%, black)

farthest-side

radial-gradient(farthest-side circle at 85% 15%, red, red 50%, white 74%, grey 76%, grey 79%, rgba(80,80,80,1) 81%, grey 83%, grey 86%, white 89%, transparent, transparent 98%, black 98%, black 99%, black)