I actually had similar questions some time ago and got some nice responses from Amal: Various questions about the flex implants and Vivokey.
According to Amal in that post, the VivoKey is expected to have the same dimensions as the FlexDF. Just keep in mind the post is some months old and stuff may or may not have changed.
I’m pretty sure Amal has his VivoKey prototype in his arm above the wrist and a FlexNT in his finger.