You don't. You pay to have it resized. The onus is on the customer here. If you buy a ring and are on a diet or new exercise regime, you should mention this. Some jewellers will offer the resizing for free anyway.
There really isn't much of a technical difference between loss of weight and loss of size. As you loose weight your size will naturally become smaller. But do keep in mind that muscle does weigh more...
Atherletics Diest :
The diet plan
Allowances per day
Half pint semi-skimmed milk for tea and coffee (115cals, 4g fat)
Unlimited fruit and vegetables as snacks or accompaniments to...
Most are done at 14ga but they can also be done at 16ga. Really, you can get it pierced up to 10ga. Length is also different for everyone. Its a good idea to have a piercer fit you with a bar....