Yeah, using basic geometry to calculate the size of a star from it's distance and the size of the dot it makes in your telescope, doesn't give sensible answers

Yeah, using basic geometry to calculate the size of a star from it's distance and the size of the dot it makes in your telescope, doesn't give sensible answers when the size of the dot is due to quantum diffraction (an Airy disc). And since that's named after a guy who lived in the 1800's, you can guess how much the ancients knew about it.