If you've already gone through the process of designing the tarp, a beak shouldn't give you any trouble.
How much coverage you desire is up to you. If you chose too small of a beak, you don't accomplish the goals of storm worthiness and privacy. If you chose too large of a beak, getting in and out is inconvenient.
The dimensions are dependant on desired beak size and tarp size, overhangs and pitch. It's your call.
Here are four ideas to consider (in no particular order)
1. A paper scale model is pretty easy.
2. You could use trigonometry and a spreadsheet to design the beak.
3. I use CAD software for my MYOG projects and therefore a beak would be just another part of the drawings.
4. I've added a beak to an existing a tarp simply by setting the tarp up and holding a tape measure in place along the invisioned coverage.
Hope this helps,