You can use VNC with a headless Raspberry Pi, but, absent a display with which to negotiate the screen resolution, X defaults something uselessly small: 720×480. To force a more reasonable resolution, edit /boot/config.txt and set the framebuffer size:
framebuffer_width=1920 framebuffer_height=1280
You can use a nonstandard resolutions, as with the 1920×1280 that fits neatly on my 2560×1440 landscape monitor, but getting too weird will surely bring its own reward. When you plug in a display, X will ought to negotiate as usual for the highest resolution the display can handle.
The System Configuration dialog has a “Resolution” button offering standard resolutions:
The shiny RPi Pixel UI bakes the RealVNC server directly into whatever handles the startup process these days, rendering all previous recommendations about forcing VNC resolutions inoperative. I found the trick of editing the config file on StackExchange after the usual flailing around.
Memo to Self: Remmina (the VNC client I use in XFCE on my desktop PC) doesn’t respond well to having the VNC server shut down while it’s connected. Fire up a command prompt, enter this:
sleep 10 ; sudo reboot
Then, quick like a bunny, disconnect the VNC session.
