"The full moon happens when the moon is exactly on the opposite side of the Earth from the sun."
For those who observe the heavens, a celestial dance takes place between Earth, Moon, and Sun throughout the year, Full Moon is an example. The Sun’s angular diameter (arcminute and arcsecond size) changes throughout the year during perihelion and aphelion as the Earth orbits the Sun. "As the distance varies, the apparent size of the sun changes, the visible diameter ranging from 32'.6 in January to 31'.5 in July.", Navigation and Nautical Astronomy, Dutton, 1926, updated 1951 US Naval Academy, p. 237.
The Moon's angular size changes too as the Moon orbits Earth. Extreme apogee 406720 km, the Moon's arcminute ~ 29.37. Extreme perigee 356371 km, the Moon's arcminute ~ 33.53 arcminute size. Jean Meeus Astronomical Algorithms, p. 332, 1991. The Sun's arcminute size can vary ~ 1 arcminute while the Earth-Moon system orbits the Sun and the Moon's can vary ~ 4 arcminute between extreme perigee and apogee points in the Moon's orbit around Earth. This celestial dance can be observed and documented using quality telescopes too. So go Full Moon
The changes I refer to here place constraints on the distance and size of the Moon from Earth, and the same for the Sun too.