Base this on the dimensions and volume of the expander.

The dimensions and volume of the expander should be your guide. If you and your surgeon are happy with the current width (base diameter) of the expander then that is the diameter of the final implant you should use. Similarly, the implant diameter can be chosen smaller or larger compared to the expander as necessary. The fill volume of the expander should be known which can help you decide which volume of permanent implant you would desire. One caution, for a low height implant, the permanent implant will need to be a little larger than the fill volume of the expander to achieve the same "apparent" size.