If a Horizon administrator has locked the guest size, or if you are using the RDP display protocol, you cannot change the resolution of the remote desktop window.
If you have multiple monitors, you can select the monitors on which to display a remote desktop window. For more information, see Select Specific Monitors to Display a Remote Desktop. You can also configure the remote desktop window to open on a single monitor. For more information, see Display a Remote Desktop on a Single Monitor in a Multiple-Monitor Setup.