It's not perfect, but there's a very close correlation to the size of the femur with overall body mass in modern animals we use to extrapolate. See the chart i

It's not perfect, but there's a very close correlation to the size of the femur with overall body mass in modern animals we use to extrapolate. See the chart in https://phys.org/news/2020-08-dinosaur.html https://phys.org/news/2020-08-dinosaur.html There's some debate over how useful this is for dinosaurs, but something that died out 10k years ago with closely related existing species is probably easier.