I see your question was posted 9 months ago. Since then Fitbit does not list devices as just small, medium, large etc.. but rather by maximum write size. Which in my mind makes a lot more sense.
When you click on the product it will show you currently available options for max wrist size.
They currently do carry the Charge 2 HR in small and large or wrist size up to 6.7 and 8.1 which according to FitBit are Large an Xl which I will bring to the moderators attention.