Hmmmm!   I thought your question was answered in the above comments.  Depends a lot on what you use it for, your type of strokes, your personal preference.

As I said above, 6 x 8 is sufficient for me.  But, also above is a comment about someone uning nothing less than 9 x 12.

You will have to decide based on what you learn from reading the comments, past and present.

“Music is harmony, harmony is perfection, perfection is our dream, and our dream is heaven”
Henri Frederic Amiel