Oh man what a question!
Needles really have no specific guidelines for length, weight or diameter. Most people make needles to cast a long way, so the bulk of the weight is situated near the rear of the plug with a counter balance up front. As you build more and more, you will begin to understand what weighting really does. I make several needles with different weights and sizes. Everything from sinks like a rock to slow sinking/suspending to floaters, each has it's own situation and sets of circumstances, so build them to fit your needs. That's part of the fun of building your own, you can make them to do exactly what you want them to do.
-Dave