To access those options, I just went to Settings> Photos & Camera> Record Video. And if you’ll note, there’s a handy-dandy approximation there (in yellow) for how much space you’ll be taking up with each configuration. So, yeah—for those of you with 16GB iPhones, it’s probably not the best idea to record 4K video, unless you plan on clearing off your Camera Roll approximately every day of your life.
And as you may have noticed from my screenshot, Apple’s default is the “1080p HD at 30 fps” choice. If you select any of the other options, your device will helpfully inform you of that whenever you’re taking videos from then on:
If you’re on the default setting, though, you won’t see any info in that spot. This is basically your iPhone’s way of telling you that you’re either filling up your storage very quickly or that you’re taking lower-resolution videos than you could be. Which is good to know in both cases!