Hmm, do we even need trig? Naively I would imagine that simple proportions would hold — e.g. same visible size @ twice the distance = object is twice as big
4x as big, since we see in two dimensions — you can judge length & height (but not depth).
So if it appears the same size but twice as far away, the the length and height are each 2x. So that would make it the size of ~4 airplanes
Not super relevant but I find this to be true when spotting a 747/757/767 vs say a 737