BIG 40" 4k monitor so I don't have to turn my head between multiple monitors (I usually keep the resolution at 2k so I have plenty of screen real estate, but nothing is too small). Keeping your head pointed straight in front of you is very important. Tilting of any kind is bad, but the worst is tilting down and to the side; an excellent way to get a pinched nerve!
As mentioned elsewhere, I have the monitor up a bit and even have it on an arm so I can rotate it 90 degrees when needed. This gives me plenty of room in front of it for a wireless keyboard with the shown wrist rest right in front of it. In my case, I have a small, desk high, cabinet just to the right of my main area (12" deep by 13" wide). I keep my wireless trackball mouse on that with a mini-gel wrist rest (same company as the big one, "innovera"). Since I'm a big guy, it feels very natural, like an extension of my arm.