If it was infinite, how did someone come up with a radius for it?
The diameter listed in the FAQ is simply the radius of the
known world. The circular area of our plane which is basked in light. There are still vast uncharted lands beyond the coast of Antarctica where the sun does not shine, where temperatures and pressures approach absolute zero, and where life cannot possibly thrive.
The diameter of our known local area can be derived from Eratosthenes' shadow experiment, which studies the area of the sun's light upon the earth. It can also be derived from expiatory distances between the North Pole and South Pole (the South Pole is a circular track of land beyond the coast of Antarctica where the magnetic field lines are vertical). Additionally, the diameter can be derived from the circumference of the equator (the inner middle circle of a circle doubles as its diameter).
It's important to note that the circumference of the Round Earth, 24,900 miles, is exactly the same as the listed diameter of the Flat Earth.