Sorry, it's too late in the night for paint. But... That's just a standing collar. Meaning... It's a straight strip of fabric. I'd imagine that it would be as long as it needs to be around the neck, and twice as wide, in order to fold it in half. Plus seam allowance.
Yeah that is right! I would just suggest that you do this aswell so it fits to the garment better:
but do the twice as wide as chilan suggested.
p.s sorry from my pants drawing!
Edit: sorry the pics so small! Basically it says 'Cut the rectangle the length and double the height of how you want your collar'
'Then cut up leaving a few mm where your neck starts to curve(basically the sides of your neck going down over your collar bone)'
'Then open up the slashed parts a few cm and stick it on another piece of paper, add seam allowance and cut out.'