Somewhere around 20ish inches maybe a little less
Believe the largest I've seen with a measuring tape was 22"
Nice to see the jags are coming out for you, maybe they'll get as active as mine since there's more of them. If you get a chance go through kolts pictures in his profile he's got some old ones of his jags swimming and he had a pretty bright light.
Something to watch out for with the jags is where they wedge themselves. When I first got mine, for the first month or so it was stuck, I didn't know I thought it was just so nocturnal like people described. I got curious and decided to get it out, it wouldn't move no matter how much I poked it. I saw it squirm from time to time so this is why it took so long. I literally had to take the dw out and hold it upside down, I used a lot of force to push it out from the other side I thought I was gonna break its nose. She was very skinny, I took the dw out. Quickly she ate and within a week was fat and coming out in the light to feed and in the morning to swim her laps.