Xfce4-settings-manager window can be maximized, resized manually but tiling doesn't work, neither by dragging to the screen edge, nor with a shortcut key. (Tiling works for other applications).
Tiling is not allowed for dialogs, so that's normal.
But that seems rather inconsitant to me.
As Andrezej three years ago already mentioned it's possible to resize dialogs in every way.
It's also possible to maximize them either vertically or horizontally by shortcut.
Why the restriction for tiling here?
Because dialogs are supposed to be short lived window, if you want the window to be a normal window, then set the type to be a normal window.
Dialogs have a type, it's set by the developer on purpose.