Here: lets make it kindergarten grade for you
We know that cities are specific distances from each other (unless you want to argue that the conspiracy is a million times larger than what is believed) we know of cities that are on each tropic line.
From city to city, following the tropic line, until we reach the first city again we can get a rough figure of how large it is.
A->B B->C C->D D->A = A->>A = distance around tropic.
Compare each tropic and notice how they are actually very similar sizes, and also notice how the equator is larger than both.