I've gone 6s -> Xs so kept to the smaller size on the X but now I'm thinking of going all the way to the Max.
One thing that's important to me is battery life. The Xr had great battery life, the 11 is slightly better, but not only is the 11 Pro Max better still based on the published specs but also, because it's an OLED screen, if you're OK with doing as much as possible in dark mode and setting totally black backgrounds/wallpaper wherever possible then, because of the nature of OLED screens, there's more scope for optimising your power usage when the screen is on vs the 11.
I do a lot of ebook reading on my phone so the ability to set my ebook reader to white text on black background and have the OLED screen not even powering up about 80% of the pixels on my screen for hours on end is a big plus of the Pro models for me.