Like I said, as many fans as your case can hold is what I would do. Your fan controller can turn them down if they are a bit too noisy for you.
Bigger fan sizes are quieter than smaller fan sizes because they spin slower, but bigger fans move more air, the trade off is power usage (which is nothing worth worrying over) and cost(maybe a $10 difference at most). Your case can hold 1 front 140mm, 1 rear 120mm, 2 120mm and 1 200mm on the side, and 2 top 200mm according to the specs page, correct me if this is wrong. I would recommend filling all of these slots.
I'm not sure if cost is a problem for you, but if you are on a budget, 1 front 140mm, 1 rear 120mm, 1 side 200mm, and 1 top 200mm will be more than enough airflow for your case. 2 200mm on the top would pull any and all hot air out of that case and the extra 2 120mm fans on the side next to the 1 200mm will be great for keeping your gpus running cool.
I don't really know too much about those fans, but if you have heard great things about them, then by all means get them. As long as you are happy with the product, that is all that matters.