mstsc /w:1920 /h:1200 /v:servername

If your computer's remote desktop listening port has been set to 33892, you should use:

mstsc /w:1920 /h:1200 /v:servername:33892

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp\PortNumber