Buying rings on the internet can be a bit of a hassle when you are not sure of your ring size. Use the following as a guide to help work out your ring size. Remember that all human beings are different and what is average for most may not be right for you.
The best way to go if you buy rings regularly is to buy a ring gauge or sizer. There are some very cheap ring gauges on eBay. Search for "multisizer ring gauge" and you might be lucky and get one for around $3-$5. I sell these when I have them in stock as do others. Simple and effective, worth every penny. There are more sophisticated ones around for more. Up to you.
You could of course,  go to a jeweller and ask. They all have ring gauges that will tell you quite accurately what ring size you are. Measure all your fingers you intend to wear rings on and make a note. Failing that, if you have a reasonable ruler (preferably callipers) you can measure the diameter of your existing ring then convert using one of the many "ring size" conversion charts available on the net.. You do have to be very accurate as the slightest mistake will make a big difference in your result. Not really recommended, but at a pinch....
I  have found that you can use a bit if string or wire to get the circumference of your finger. If you can get a length of solder (use in electronics soldering), it works even better because it's soft, yet keeps it's shape. The length will be a measure of the internal diameter of the ring. Make sure you take into account your knuckle when you do this AND the fact that your finger is not round.  Again, not really recommended, but at a pinch....
There are MANY printable ring sizers available on the net.   If  you Google 'printable ring sizer' you will find many. The Ebay guide on ring sizes also has one. These aren't too bad.

POINTS OF INTEREST. Just because you might be a small person, it does NOT mean you have small fingers or vice versa.
The most common size for a woman is M to Q  (6 to 8). For a man it is  R to V (9 to11)
Your ring size is NOT constant. Time of day or year, health, eating, hydration, temperature and many other factors can affect your fingers.
If you are buying a wide band as opposed to a thin one, experience says you should add about a 1/4 to 1/2 size.
If you are right handed then the corresponding finger on your left hand is about a 1/2 size smaller and vice versa.

ON RESIZING RINGS. To make some rings larger by a small amount is relatively easy for a jeweller. The band at the back can be hammered or stretched a little to increase the size. BUT be aware some rings cannot be enlarged this way or in any way. If you have inlaid stones, enamel or the shape is awkward, you risk damage. Resizing is best done on plain rings or bands.
The other way a jeweller changes the size of the ring is to cut the band and either remove or add a piece. To do this he has to heat the ring which can cause all sorts of hassles. Some stones have to be removed and put back. Some are almost impossible to remove and many stones cannot handle heat. If you have a pattern or stones all around the ring, you will lose some detail and symmetry.
Your best rule of thumb is to buy a ring that fits, or be prepared to wear the ring on a different finger. Most people have 8 different sizes on their hands. An alternative, if a ring is a few sizes large, is to buy a gold 'size reducer', available at most jewellers. It clips inside the band and makes it smaller. Not an ideal solution but useful. Personally, unless you have a lot of band to work with with a simple setting, I'd leave resizing alone.

CHAINS & NECKLACES. Fortunately chains and necklace are a lot easier than rings. All it takes is a tape measure for you to get an idea of the length you require. Just bear in mind that if you are buying larger beads etc,  you will have to add at least an inch or 25mm to its length  length to compensate for the larger diameter of the necklace. It is normal to measure from the hollow at the base of the neck. If your neck is 15 inches or 37.5cm then a 18 inch or 45cm chain will hand 1 1/2 inches or 3.75cm below the base of the throat. Some of the common lengths are listed below.

Collar length:  Around 13 inches or 32.5cm. Too tight for most people.
Choker length: around: Around 14 to 16 inches or  35cm  to  40cm. as the name implies , very high on the neck.
Princess length: Around 17 to 19 inches or 42.5cm to 47.5cm. A common size for the base of the neck. Great for pendants etc or open neck clothing.
Matinee length: Around  20 to 24 inches or 50cm to 60cm. This falls below the throat. Good for  high neckline clothing  when dressing to impress.
Opera length:  Around  26 to 34 inches or 65cm to 85cm. Long enough to go straight over you head. A clasp is still a good idea in case you want to double it around you neck. Very versatile.
Rope length: Generally greater than 40 inches or 1 meter. Not common but very versatile when wrapped around a few times.

BRACELETS.  Bracelets are a little bit more difficult than necklaces or chains. You can use a tape or a ribbon to measure your wrist size, but you don't want it tight, so you'll have to add  a little to be comfortable. If you have large beads or links you have to add even more to the length to compensate. OF COURSE the best way is to measure is to use an existing bracelet you are comfortable with.

As an average use the following:
SMALL is about 14cm or 5 3/4 inches when you measure your wrist. You need a bracelet about  17cm or 6 3/4 inches. That's adding an 2.5cm or an inch for looseness.
AVERAGE is about  15cm to 17cm or 6 to 6 3/4 inches when you measure your wrist. You need a bracelet  about  17.5cm to 19.5cm or  7 to 7 3/4 inches. Again adding about  2.5cm or an inch.
LARGE is about 17.5cm or 7 inches or more when yo measure your wrist. You will need a bracelet  20 cm or 8 inches or more when you add 2.5cm or an inch.

When adding to the length of the bracelet for looseness we added 2.5cm or 1 inch above. If you add ....
7mm to 12mm or  1/4 to 1/2 inch you should have a tight-ish bracelet that won't move very much.
18mm to 25mm or 3/4 to 1 inch you should have a dangley type. A comfortable normal size.
31mm or 1 1/4 inches you will have a loose bracelet or for one with large beads.

LASTLY, I point out that I am not a jeweller and that the above guide is just that, a guide. If you want professional advice and someone take responsibility for recommendations, go to a jeweller. I am only a pawnbroker with 24 years experience. SEE MY OTHER GUIDES FOR OTHER  INFORMATION ON  JEWELLERY. This is all from an Australian perpective. Edward.