Targeting the console - screen :0 is possible but will create other issues.
I believe we have been around these topic several times.
Never use screen :0.
As Plasma based systems starts the greeter on screen :1 and the session on screen :2 these may create issues as well - at least my testing has shown those unstable an error prone.
The only method I have been able to make functional with no errors down the road is [root tip] [How To] TigerVNC Quick Setup which I created about a month ago due to the members having issues with screen :0
Always set the geometry in the user account’s configuration - whether this is /root or /home/username - as @cscs point you to.
From x0vncserver(1) — Arch manual pages (Parameter names are case-insensitive according to x0vncserver -h)
-Geometry geometry
This option specifies the screen area that will be shown to VNC clients. The format is widthxheight+xoffset+yoffset** , where+' signs can be replaced with-’ signs to specify offsets from the right and/or from the bottom of the screen. Offsets are optional, +0+0 is assumed by default (top left corner). If the argument is empty, full screen is shown to VNC clients (this is the default).