$leter='c:'
$valueToReturn="TotalSpace"

If($valueToReturn -eq "FreeSpace")
{
Get-WmiObject Win32_logicaldisk -Filter "DeviceID='$leter'" | select FreeSpace
}
ElseIf($valueToReturn -eq "TotalSpace")
{
Get-WmiObject Win32_logicaldisk -Filter "DeviceID='$leter'" | select Size
}
Else
{
Get-WmiObject Win32_logicaldisk -Filter "DeviceID='$leter'" | select Size,FreeSpace,@{n="Used";e={$_.Size - $_.FreeSpace}}
}