This article includes a 1:22 video walkthrough.
Quick answer
Needing a footrest isn't about your height. It's about your setup. Tilting the chair lifts your heels, and raising the seat to match your desk lifts your feet further. So whether you're 5'6" or 6'0", you may need a footrest to keep your feet firmly planted.
How to tell
Get into the tilt position you work in. Are your heels still firmly on the floor?
Pull up to your desk and set your seat so your forearms are level with the desk at 90 degrees.
If your feet are now skimming or off the floor, you need a footrest.
Also called: do I need a footrest, tall people footrest, feet off the floor, foot support.
Video transcript
Needing a foot support or not isn't about your height. It's more about your setup, and I'm going to show you what I mean by that. For reference, I'm five-six, and I've got the chair in the lowest position. My feet are firmly planted. So I'm good, right? I shouldn't need a foot support. Well, I like to sit in tilt, because that's what we recommend at Anthros. So when I tilt, look at what happened to my heels. So even if my desk was correct in this position, I would still need a foot support, because my feet aren't firmly planted.
When I go to my desk, again, I want to be correctly oriented at my desk, so my desk height should be the height of my forearms in this position, and I'm a little short. So I need to raise my seat height, and that brings my feet completely off the ground. So for sure I'm going to need a foot support.
So you can see, even if you're six feet tall (and I get this a lot: "I don't need a footrest, I've never used a footrest in my life"), because of the tilt and possibly your desk height, you may be six feet tall and need a footrest. Take these things into consideration when you're deciding on a footrest.