need to replace iring from control box to valves, what size is exceptable?

This is the wiring guidance given in the timer manual: Valve wire: direct burial, color coded multi-strand • 18 gauge for runs less than 800 feet • 14 gauge for runs greater than 800 feet 800 feet should be a long way so 18 gauge will most likely work just fine, and it can be found on the Lowes.com site by searching "sprinkler wire" in 50 and 100 foot lengths.