As Johan correctly said, it largely depends on your client's needs, whether they're intent on printing or just digital use.
With print, anything goes, but obviously going larger than current common crop of high-end fullframe cameras is unneeded. That would be 40+ Mpx or 8k.
With digital, common format is still 2k (and most common fullHD 1920px, or 2048px). But times are changing, even phones are getting quadHD (2560+ px) or even 4k displays. Tablets/Laptops are 3-4k.
So do expect clients to ramp up their expectations even when they want pure digital output in their campaign.
I've been personally rendering 8k for years, I am in love with resolution. It can't be appreciated in digital format sadly right now, but in print, it's amazing. The lifelike detail and tactility, you want to touch it :- ).
And no fear of overkill.. a client for whom we do product design renderings use PhaseOne XF 100mpx, making the most unreal 12k detail I've ever seen in photography. There is still long way to reach that.