The size of your Shell's screen should match your browser size, like in this screenshot:
However, there are some cases where they won't match, or where the Shell desktop size won't be updated at all.
Here's software that isn't currently compatible with how we set the screen size:
This is a popular desktop environment used by a number of different distros and images out there. SPICE doesn't resize the screen in the way that Xfce expects, so the screen size is not updated on this DE. There is an upstream issue in Xfce that discusses implementing this functionality, but we're also investigating other possible solutions to this. In the meantime you can set the screen size correctly by either logging out and back in (when available) or with an xrandr call.
This is an alternative to Xorg. SPICE was written to support Xorg, and doesn't play too nicely with Wayland. When using Wayland as your display server, you may end up with broken-looking or blank space to the bottom and right edges of your screen. Right now, Xorg generally seems to be more performant on our setup. Disabling Wayland can provide you with a better experience in the meantime, but we're investigating possible solutions to fix this properly.