If you know how far away it is, and the angle that it subtends, you can tell how far away it is its size. For example, if something is 100 meters away and it fills 10 milliradians, it's 100 * 0.01 = 1 meter across. (A milliradian is 1/1000 of a radian, and a radian is about 57 degrees.)
The size in your telescope is easy, and it's a simple optical calculation to figure out how large an angle it actually takes up. That is, so long as it's something that appears as an object, not just a point. So the method will work with a galaxy, but not a black hole
If you have the better answer, then send it to us. We will display your answer after the approval.
Rules to Post Answers in OneStopFAQs.com:-
There should not be any Spelling Mistakes.
There should not be any Gramatical Errors.
Answers must not contain any bad words.
Answers should not be the repeat of same answer, already approved.