If the Key Line width on the final hi-res produced product/file does not appear to match what is shown in ProSelect, then this can be due to using a keyline size that is too small in your applied style.

Because a key line size set to less than 1 pixel in width on a computer screen would normally be invisible, ProSelect automatically increases the displayed size of these to 1 pixel so they are visible. 

Make sure that you set the actual size that you want on the final product (in Edit Styles under the Templates menu).