The only good way to estimate how big they get is to look at the parents. I've never seen a baby be more than one inch shorter or longer than it's parents - unless the parents are of significantly different sizes.
Veileds can be as small as 11 inches total length to as long as 26 inches. Most of the small ones you see are small because of nutrituional problems - but there are wild populations of very small individuals. Similarly, not all veileds can get up to 20 inches. Those come from certain populations.
Seaside reptiles has some WC coming in from Arabia, and in the group are several 17" females and 26" males. I've seen a 25 inch male veiled, and it was a monster.
Veileds tend to be less robust than panthers, and are probably lighter on average. No panther's going to be as big as the biggest veiled, but most panthers tend to be bigger than most veileds, if that makes sense. Panthers vary less in size.
on average, veileds end to be about 15" long.