I doubt that pot is actually 8.5 inches in DIAMETER. It's likely 8.5 inches across. Being more like 20 inches in diameter.

I started in what is called a 6" pot. It held about 1/2 gallon (maybe). My second pots were 10 inches across and likely held just OVER a gallon.

So, most people end up in a 3-4 gallon pot by harvest. That should work fine for you(if space isn't an issue, like growing in a short box). My 4 gallon pots are 14.5 inches across(not diameter) and 12.5 inches tall. If that helps you figure that out.

Sorry I just can't find my diameter tape measure. I'd be far more accurate if I could.