Length really does not matter as long as it's not ridiculous. They'll add a touch of weight to back of your arrow, but not enough to concern yourself with.

I personally use a 5" wrap with Blazers. However, I just got this brainy idea to add some designs to mine and may go with 7" for that "bling" factor.

The larger the wrap, the more visible they'll be in the woods during the shot which may help determine where you hit an animal.

As for overlap. I make all of mine 1" wide. This creates about 1/16" overlap on a Goldtip XT Hunter shaft. When you stick vinyl of vinyl, it creates a better bond than when they're directly on the shaft itself. Depending on what arrow you're shooting, you may have to experiment with the width to get them right. But I think 1" is a good starting point for most shafts with the exception of the slim shafts like the Axis arrows.