system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know the size of planets in our solar system and how far away they are?
gpt
We can measure their position as they move through the year.  Earth goes from one place to another, and the motion of other planets can be measured relative to the fixed background stars.  It's a lot of math to reduce that to their orbital parameters, but Johannes Kepler did that math in the 16^th century.

We've gotten good size measurements be sending spacecraft to visit every planet + Pluto in our solar system.