Shaq looked like he had a decent amount of headroom left once he got settled in.I think anybody of any height or weight should be concerned. I am. None the less, would you feel more confident about fitting comfortably in a Rivian if I told you the 7' 1", 300# Shaq fits comfortably in one? Well, he does. Check out this FoxNews article with an imbedded Instagram video of him getting into a Rivian. Plenty more examples on the web if you search for Rivian and Shaq.
If there are none of them have been announced publicly or to the pre-order holders. Your best bet is to contact Rivian directly and ask them - [email protected] or via one of their social media accounts.