Twist rate calculator http://www.uslink.net/~tom1/twistrate.htm
The Hornady 250gr bullet is 1.27" long...So if 250gr is the longest you want to shoot then a 1:16 would be fine...Esp.if you were also going to shoot cast boolits as they like slower twists at higher vel...

Barrel length..That would depend on your use and what you like...