If this is your first grow I reccomend something like lowryder 2 or some kind of automatic strain. 100% Sativas are pretty hard to grow inside. Especially in a confined space. But if you want to grow it it will take a lot of manicuring and care. You will have to flower them before the are fully mature so the smoke won't be as good. But hey who am I to tell you not to. What size to start flowering? Well 3rd or 4th node maybe. Your biggest problem is height. You said you have 61" Well take 24 of that away because the light needs to be about 12"-18" above plants and then you have the thickness of your light. So in reality you are only working with 37" And that is waaaaaaaaaayy small for a 100% sativa.