how big should my res be??

building my own hydro kit, only doing 2 plants, limited on space, didnt know how big my res should be....and where can i get a timer that will let me use the pump 1 min on 5 min off...or the other way around, sorry...newb. any advice is appreciated

also...should i do just a pump running up to the 2 grow nets, with air stones on the bottom, or how should i set it up....need to know how many GPH i should be pumpin, and cycles for pump