I still need to double check under my car. When I got home from work yesterday and changed clothing, I was too tired to check. I think the sway bar on the X5 5.0i comes stock based on the order sheet and I thought I read somewhere that it's a 26mm diameter but my memory on this is fuzzy. I'll look for that thread and will post a link if I can find it. I think 24mm would be good if the person doesn't have anything but bigger is better on this item imho if there is indeed a 26mm option out there.

EDIT: I updated my earlier post. There's definitely a sway bar and it looks to be ~ 26mm.