Don't be pedantic. You'd be able to see enough to judge their dimensions. I obviously don't need to be able to see all parts of a tennis ball at once to be able to measure that its diameter is about 2.63 inches.
Except that your estimates of the diameters of the sun/moon rely on knowing the distance to them, and oddly enough the answer to that question always comes up differently depending on who's doing the calculating and where they're doing it.
On the earth's distance from the sun Copernicus computed it as 3,391,200 miles, Kepler contradicted him with an estimate of 12,376,800 miles, while Newton had asserted that it did not matter whether it was 28 million or 54 million miles 'for either will do as well'.
newton said some crazy stuff.
By the way, what is the context for newton's assertion?
Tom is - as usual - taking Newton out of context. Newton was talking about a calculation involving ratios, for which the goal did not require an exact number, merely to know the ratio of the earth to sun distance and another distance.
You may be interested in a thread I started a few months ago discussing Ole Romer's measurements regarding the speed of light and hence the calculation of the size of the solar system. It contains some classic FE fails, such as claims that Romer's original measurements were lost and therefore he might have been wrong, which I countered by showing where an image of his measurements in his own handwriting could be found. Levee is also in it for entertainment value, IIRC.