How to size clogs?

I am going to attempt felted clogs for christmas gifts. My intention was to start with and experiment on the hubbies pair, but LYS sold out of the adult pattern so I will start with niece and nephew. How do I know what size to make them? Their feet measure 8 1/4" and 6 3/4". Should I just go up one size? I want to be sure I have room to felt them completely so that they will be washable in the future.

I think you’re better off a bit too big than small–especially for kids. I’d probably go up a size.