W a 1k, you can easily get away w a 6", maybe even 4-5", depending on a lot of variables... the actual ambient temp vs the temps of air from your draw area, what else you got running in the room, how you run the lines (bend/corners = significant reduction in air flow), & where you place the fan (pulling or pushing). A lot of small things that make a final impact to your temps...
I would go w a 8" which will allow for you have all the issues above covered. It will likely also allow for you to add a second light. Would get a Speedster controller to dial it down, should that be necessary. W fans... always better to go bigger & adjust down vs having something too small for the job. Peace.