What size transformer do I need to power a ¾" 24VAC solenoid valve?
3/4" solenoid valves use around 20 to 30 VA. to open. Look on the name plate and it should say what the  VA. or watts is. You can buy a ( 120/208/240) input X 24 volt output transformer Rated at 60 va.for about $50.00. I saw on  Amazon  a Siemens MTO100C 120/208/240volt X 24 volt, 100VA. for about the same price.