Depends on how high you want it to grow. That is the beauty of indoor grow.....
Plants will stretch up to 3X thier veg height when put into flower. So you make the call on that one Just plan for 3X increase and you will be just fine.
As for your lights....you need to be able to adapt your enviro around the plants as much as your plants around the enviro.
Give and take.
Ideally, you should leave it at the top the whole time. You can turn the plant as you need it.
You can supplement with side lights and lower lights in the form of floro or CFL.
Any posts made by me are purely fictional in nature and by no means is anything I say to be taken seriously. I do not grow or condone the growing of anything not legal. Any and all pictures I post are pictures widley available on the internet and any discussions I am involved in are purely hypothetical or are commentary in nature and should not constitute advice or be considered advice to assist in activities that are deemed illegal.