I didn't find any command line arguments reference for the new Microsoft Emulator for Windows 10 Mobile. It is basically a simple Hyper-V VM client which creates and plays virtual machine from a Windows 10 Mobile VHD image. It also displays and manages its console and provides some other interface options.
It is probably supposed to be part of Visual Studio 2015 but I often do not want to have such a big deployemnt. You can download its separate installer from the Downloads and tools for Windows 10 page (scroll down to the Other download options section). Although the installer is just about 1 MB large, it will download some 4 GB online, especially the VHD OS image.
After installation the image goes in C:\Program Files (x86)\Windows Kits\10\Emulation\Mobile\flash.vhd while the the emulator is installed in a versioned folder under C:\Program Files (x86)\Microsoft XDE and started as XDE.exe. When you start it with /? parameter, you will get a GUI parameter reference. I was able to extract the reference in a pure textual form with the SysInternals strings tool.
So it comes here:
Emulator version 10.0.10586.0
xde [/name vmname] [/vhd vhdpath] [/memsize sizeMB] [/com[1|2] pipeName] [/createDiffDisk diffVhd] [/language lcid] [/video widthXheight] [/diagonalSize screenSize] [/showName] [/noStart] [/snapshot] [/displayName text] [/fastShutdown] [/noALS] [/noFFC] [/noGyro] [/noMag] [/noNFC] [/softButtons] [/camera capability] [/noGPU] [/?]
/name vmname: The name of the virtual machine. If you do not provide a name, the default 'Default Emulator' is used.
/vhd vhdpath: The path to the VHD for the virtual machine (required if the virtual machine does not exist).
/memsize sizeMB: The amount of RAM in MB that the virtual machine should use. Default: 512
/com[1|2] pipeName: The named pipe that the host uses to communicate with serial ports on the virtual machine.
/createDiffDisk diffVhd: Creates a differencing disk, using the path specified by /vhd as the base. The virtual machine uses diffVhd for its VHD.
/language lcid: The hex language ID to use for the emulator. This ID applies only to the host-side emulator application and not to the guest. Example: /language 040c
/bootlanguage lcid: The hex language ID to use for the emulator. This ID applies only to the guest-side OS and not to the host-side emulator application. Example: /bootlanguage 040c
/video widthXHeight: The initial screen resolution of the emulator application. Example: /video 640x800. Default: 480x800
/diagonalSize screenSize: Specifies the diagonal screen size for the emulator (in inches).
/showName: Displays the name of the virtual machine on the emulator chrome.
/noStart: Exits the program without showing the emulator (after creating or modifying the virtual machine).
/snapshot: Tries to boot to the default snapshot, or create a new snapshot if no snapshots are found.
/displayName text: The text to use for the application window and task bar icon.
/fastShutdown: Turns off the virtual machine without waiting for the OS to gracefully shutdown.
/noALS: Disables the Ambient light sensor.
/noFFC: Disables the Front facing camera.
/noGyro: Disables the Gyroscope.
/noMag: Disables the Magnetometer.
/noNFC: Disables the NFC sensor.
/noGPU: Don't use desktop accelerated graphics.
/softButtons: Enables the software buttons. (Not valid for all resolutions.)
/camera capability: Switches the camera to the specified capability mode. Valid capabilities: WP8
/?: Displays this help dialog.