Since the standard adventruing wizard can use a Kobold like a soccer ball, I would say about 1.5 lbs. Maybe 6 lbs with all their armor and other gear.
"Wizard looks to the left, looks to the right and kicks a Kobold right in the , its a good pass to the Fighter who kicks it into the Shaman! Its a GOAL, A GOAL! This was a spectacular game!"
On a serious note, a kobold is about the same height and weight as a halfling. So with them having comparable mass, use the halfling height and wieght range from the PHB.
I just weighed our family kobold and he weighs 25 pounds, but he's a bit overweight and we're putting him on a diet, so you might want to round that down...
(Oh, wait, that was the family dog, not a kobold after all, so never mind.)