Your surgeon has chosen to place the new implant under the muscle. This is a wise choice for several reasons.
First, if the implant is placed under the muscle, it has an extra layer of protection which will make the implant feel and look more natural than if placed above the muscle.
Secondly, you will eventually need to have a screening mammogram. It is much easier to see the breast tissue if the implant is placed behind the muscle.
However, if the implant is placed behind the muscle, it will not fill the breast as much as if the implant is placed above the muscle. Therefore, I think it is reasonable to go slightly larger. If you were happy with the 230cc implant above the muscle, then 30-50cc larger will be required to maintain the same look.
300cc implant size is certainly reasonable, given your height and weight. But it really depends on how conservative you want to be, and on your chest width, height, and amount of breast tissue.
If you have a lot of loose skin and your nipples are to too low, you may need a breast lift to tighten and reposition the skin. A larger implant alone may not be sufficient to correct the laxity of the skin.