where's X-Crispi when you need him? He'd be the perfect one to ask. I believe he uses 3/4" tubing for his and likes it much better than the 1/2 inch.

Even in my larger buckets D4 I'm perfectly happy with 1/2 inch tubing so I wouldn't be the right one to ask.

I suppose though, I would ask how you are going to recirculate....with an air pump or a water pump? That may change how large you need your tubing to be.