#$arg1=$args[0]
$arg1 = "C:\myfile\file.ext"
$drive = (Get-Item $arg1).PSDrive.Name

Get-WMIObject Win32_Logicaldisk -filter "deviceid='$drive`:'" |
Select PSComputername,DeviceID,
@{Name="SizeGB";Expression={$_.Size/1GB -as [int]}},
@{Name="FreeGB";Expression={[math]::Round($_.Freespace/1GB,2)}}