Please give more info about your set up. If hydro, I would say go with a 5 gallon rez minimum. This will be dependant on other factors like temperature as well. Also, are you looking to have to top off the rez everyday? or do you want some room to slack a little? If you don't mind topping every day towards the end of flowering, you can stay pretty low on the rez size (3-5 gallons at the lowest). If you're a lazy stoner like me, maybe try an 8 gallon rez so you can miss a day of topping off or so without the rez going dry. Again, rez size will change based on whether you're doing DWC, ebb n flow, or hempy bucket. Just figure out how much you're willing to put into this grow of yours and utilize the appropriate method. We'll be here for any ?s you run into boss. good luck.