You can use Horizon Client with external displays and you can change screen resolutions.
When you connect your Chrome OS device to an external display or projector, you can display Horizon Client in full-screen mode by pressing the full screen key on your device's keyboard.
Enlarging the Screen Resolution for a Remote Desktop
By default, the display resolution is set so that the entire Windows desktop fits inside your device, and the desktop icons and task bar icons are a certain size. If you change the default to a higher resolution, the desktop still fits inside the device, but the desktop and taskbar icons become smaller.
Changing the Display Resolution Setting
To change the resolution setting, tap the Settings (gear) icon in the upper-right corner of the Horizon Client screen, tap General settings, and tap Resolution.
Screen Resolutions for Using Projectors
You can use the Resolution setting to set a larger resolution for projectors.