Tribalectic has some great, inexpensive CBR's. I would get 3-4 different sizes and see what's comfortable. For instance, if you think 5/8" will work, get 9/16", 5/8", and 11/16". Or if you're REALLY not sure and 5/8" looks close, get 1/2", 5/8", and 3/4". Those would either be in 1/16" increments or 1/8" increments. Remember, though, 1/8" is actually a huge difference when it comes to jewelry.
There's no harm in having the extra jewelry around. You may find another hole for it a different day.
If you just want to buy one piece, you'll probably need someone with experience to choose the diameter. That usually means a trip to the piercer!