I have oregano (I am not sure if it is Greek) in a small herb raised garden. It is now the size of a small bush, little over a foot wide. So I am thinking you should look for a pot that would be at least 1.5 foot wide.
Hope this helps.
It depends on how much oregano you want to have around. In the ground it is a spreading mound. I only need a little so I keep mine in a 6 inch pot and I keep dividing it to make more.