Here's something, if you doubled the diameter of the sun the volume would be multiplied by 8, but if you double the distance from it the amount of light needed to illuminate it would quadruple. As it gets bigger the volume to area ratio increases, as it gets smaller it decreases. The sun needs to give out more than 100 peta watts to illuminate the area of the earth it illuminates. If the flat earth sun was 32 miles or 51.5 kilometers in diameter. It would have a volume of approximately 71,500 cubic kilometers, 71.5 trillion cubic meters or 71.5 quadrillion liters. We'll be generous by using the 100 petawatt figure, the real figure is significantly higher.
There are 31,536,000 seconds in a year. This gives us 3,153,600 exajoules per year, or 3,153,600,000 QUADRILLION joules. This gives more than 44 million joules per liter annually. TNT only has 4.2 million joules per kilo and 7 million joules per liter! If the Flat earth sun is as dense as water than it would need an energy density more than 10 times higher than TNT just to produce the reduced figure for a year! At 20 times the density of water or a density nearly equal to osmium, the densest element, and a energy density 50 times that of TNT. THe flat earth sun would only be able to produce the reduced figure for less 100 years, there are people older than that! For 4.5billion years at the reduced amount the density (Kg/l), times the energy density (In TNT) would be 47.5 billion! It the average density was less 300 times the density of water Even fusion would not be enough. Therefore antimatter matter reactions would be necessary, this would have an average density of more than twice that of water. Then there would be the problem of of a lot of the light being x-rays and gamma rays, as not enough of it would be absorbed, a single gamma ray for an anti-matter matter reaction is enough to heat an atom up to temperatures where it will emit lower energy gamma rays, these can heat up an atom to the point that it will give of gamma rays an x-rays, that's just one. If the FE sun used anti matter-matter reactions then it would likely emit large amounts of ionizing radiation. Especially because higher gamma rays need more to intercept them., and the gamma rays given of by proton-anti proton reactions are very, very, high...