Yes, they have a very good idea on accuracy. For the record, I was originally a young earth creationist but curiosity got the better of me. Geologists are very obsessive about rocks so it’s just a case of puzzling out what rocks match perfectly despite distance, accounting for things like erosion or volcanoes, and lining up the various ages of rocks for when they were made and when other rocks filled in. Then add fossils to check connections and weather conditions. There is obviously more than this, but it’s something a person can get real deep into.

The planet would not be noticeably different in size. The last time the earth got a noticeable rise in mass we ended up with a moon. There is evidence of objects in the lower mantle that are most likely old sunken tectonic plates, but I don’t recall details.