I have not had a chance to set it up outdoors and take a pic.
Mine is a little more than just a beak. I added 3' to the length and also the flat beak. Its basically 5x12 with a 2'6" point in the middle/IE not permenantly sewn into a drop.
Here is a link to a sketchup of it.
In this form it has a lot of room. 5x11 would be good for most people. I am tall.
As for adding one onto a 5x9 tarp/poncho if you could imagine this guys setup, then you have a 2'6" pointed beak following the line going out. I folded mine over and have multiple attachement points so I can vary the pitch of the beak.
With a factory tarp it would be somewhat tight, but light. With mine at 12' long I have loops on the back edge at about 15" in from the corners and fold under the flaps. I peg that then rotate the full front corners around to that width. That gives you about 8'6 long x 5' wide x 3'8" high. It could be pitched closer to the ground, with the beak almost touching the ground for more protection.
I also thought about just making a triangle piece to zip in on the open side of the factory tarp. I have not figuerd out how big it would have to be, but that way you could have a zippered entry at the corner.
If you are going to use a longer one, its probably easier to make one from scratch. 2nds sil is cheap and you could make it 5'-2" 5'-4" wide, full width. 4 yards would cost about $28, plus another 2 for the beak $14.