If by smaller you mean standard 70mm, yes, the screen will be shorter because of the aspect ratio, and probably narrower, unless your local 70mm theater is a former palace with a 60-foot or wider screen. But despite that, 70mm film has higher resolution than any digital IMAX system. And IMO, the film experience is better. Nolan shot the film with all formats in mind, especially standard 70mm, so it won’t be framed “wrong” in the 2.20 ratio, just different.
The third-best experience the dual-laser IMAX “GT” system you mentioned. The problem is that IMAX Corp. and the theater chains don’t make it easy to figure out which of the several IMAX digital systems your local theater has.
Here is a video that accurately outlines the different IMAX systems. It’s cued up to the section about the various systems. You might also find the earlier sections interesting.
AFAIK, there are no theatrical 8K projectors. They are in development and may be introduced next year, but they probably won’t become widespread because exhibitors are losing business and probably aren’t inclined to make major investments in new equipment. It would also be much more expensive to produce films in 8K or higher.
I worked for 37 years in the IMAX business, and although I’m no longer in touch with the company, I’d be surprised if IMAX is planning to offer an 8K system.
Nitpick: they are “shot for IMAX”, but practically speaking, that’s just marketing BS. Those “IMAX-certified” cameras are just high-end digital cameras that are also used to shoot non-IMAX movies. There are no special digital IMAX cameras.