Complex sentence: The exact size of "Paraceratherium" is unknown, due to the incompleteness of the fossils, but the weight is estimated to have been 15-20 tonnes at most, the shoulder height, and the length about.
Split into simpler sentences: The exact size of "Paraceratherium" is unknown because of the incompleteness of the fossils. Its weight is estimated to have been at most; the shoulder height was about, and the length about.