The Safari Tuff tab is designed to give you an indication if you cup your hand. I think that’s what the extra length is for. The correct width of a 3 under tab is the width of your three fingers held together, no more, no less. The retaining loop should be on the finger bone that connects to your hand of your middle finger. I think it rides more or less in the middle of that bone, but I have shot it so often that I don’t really try to position it. The important thing is that when it is the correct size, you should be able to see 1/4-1/2” of fingers sticking out in front of the tab when you are wearing the tab with your fingers extended. When you position the bowstring against the raised ridge in the tab, you should be able to curl your fingers in a deep hook, and get a consistent string grip every time you shoot the bow.
With a simple tab of a piece of leather, you can trim it yourself to fit. But with a fancier tab like the Safari Tuff or Bateman, it is important to buy one that fits correctly.