I have an image about 1000 by 600px.
In Photoshop CS5 I could set the pixel width of the line to say, 3 px and drag the mouse from point A to B to create the line. Now, in CS6with the vector system, I cannot set the WEIGHT of the line to less that .35mm. I cannot create a thin line!!
Grrrr
Try using the shape tool. Just tested it in CC and you can set the line as small/thin as you'd like.