What I've always done is:
Measure how many inches you want the collar to be.
Place Markers one inch apart on the fabric.
Say your gauge was 5 stitches per inch, you would knit up 5 stitches between the markers. Same with any other number, like if you got 3 stitches per inch, knit up 3 stitches in between the marker, and so on.
I design my own sweaters, but if you have a pattern, it will tell you how many stitches to pick up. That's how I've always done it with my sweaters. Hope that helps.