Let's make things clear : when you purchase something in SL, you don't want to spend three hours adjusting it. There will always be some skills and work needed to adjust complex multiprims objects as piercings for example since (luckily, cos I don't like clones) each shape is different...
But for nails and rings, there is a way to save you time and energy and a bunch of hand jewelry & accessories understood it and now use this system.
You may have noticed it, if you're a shopaholic and if you love accessories as much as I do. Most of the hands jewelry & nails are sold in most of the time three hands sizes : 10,20,30.
For example, in some of my folders you'll find this : [VIRTUAL/INSANITY] HEPING Rings & Nails R20 and [VIRTUAL/INSANITY] HEPING Rings & Nails R10.
Concretely, what do these number mean ?
Numbers actually represent the hands size on your shape.
You'll find this by going on appearance mode, chosing torso and hand size like on the pic.
Why 10, 20, 30 ? Because these are the most used hands size for girls.
What happends if your hands shape isn't on 10, 20 or 30 ? Then your hands will be too big or too small and you'll have to adjust the rings and nails to it on your own when actually, if your hand shape is 10 and you wear the size 10 rings & nails, your rings & nails will fit PERFECTLY directly !
It means that you'd better think, if your hands size is 18, to switch for 20 so it saves you work and time.
To me, as a builder, it takes me more time to adjust three hands sizes than to create one size only but it is just made to help you. So really, take profit of it. :).