She is plenty big enough to breed, and good sized for a 3 year old. Don't overfeed her as a fat female can have issues trying to pass the eggs. Her body mass looks good, as is.
As for calcium supplements I used SuperPreen with OsteoForm for years. Just make sure that you are providing vitamin D3 along with the calcium.
Oh yeah, you DO have a male to breed to her, don't you? That could be important.
