what is the longest barrel on a charger that can be used for unlimited?
is it better than a standard 10" barrel?

15" is the maximum barrel length allowed in Unlimited.

Length of the barrel is less important than the quality of the barrel.

I had a Charger that I put a longer aftermarket 14 7/8" bull barrel on. Way too heavy. I then got a standard weight 10/22 rifle barrel and cut it off to just under 15". Way better as far as weight goes. Don't know if the accuracy was better with the longer barrel but longer barrel length was better when shooting creedmore. Any 10/22 barrel will work on the Charger as long as you don't exceed 15" for unlimited.