Please use the following steps.
1) Select a picture on the wall
2) Step back from it.
3) Hold your hand out at arms length.
4) Close one eye, and hold up you little finger, adjusting your distance from the picture on the wall so that your little finger just blocks out your view of the entire picture.
5) The angle subtended by your finger and the angle subtended by the picture, as seen from your eye, are the same.
6) Let s = the width of your finger
7) Let r = the horizontal distance from your eye to your outstretched hand
8) Let S = the linear width or linear diameter of the object on the wall
9) Let R = the distance from your eye to the wall. Then the following relationship is true.
10) Measure S in cm
11) Measure R in cm
12) Angle subtended by your little finger = S/R x 57.3 = ? in degrees
13) Now use your little finger to measure the angular diameter of the Moon.
14) Does your little finger covers the Moon? How much of it will cover the Moon?
15) By knowing the angle the Moon subtends and its distance from the Earth, can you find the diameter of the Moon?