hi there, rainbows always happen at the same angle, caused by the refracted light through water droplets. (it’s about 42 degrees I think) So the size of the one you see is probably determined by your distance from the water droplets… someone further away might see a bigger one. But that’s just a bit of a guess..
This is determined by the raindrops that make the rainbow.
A typical raindrop is spherical and therefore its effect on sunlight is symmetrical about the center of the drop and the sun. The symmetry of the focusing effect of each drop is such that when we view a raindrop along the rainbow ray, we will see a bright spot of reflected/refracted sunlight. We know that the rainbow ray for red light makes an angle of 42 degrees between the direction of the sunlight and our eyes. Therefore, as long as the raindrop is viewed along a line of sight that makes this angle with the direction of light, we will see a brightening. The rainbow is thus a circle of angular radius 42 degrees.
We don’t see a full circle because the earth gets in the way. The lower the sun is to the horizon, the more of the circle we see – right at sunset, we would see a full semicircle of the rainbow with the top of the arch 42 degrees above the horizon. The higher the sun is in the sky, the smaller is the arch of the rainbow above the horizon.