You got it about 95% of the way there. I would do:
Get-NcVol | Where-Object {$_.State -eq "offline"} | select Name, @{l = "TotalSizeGB"; e = {$([Math]::Round($_.TotalSize / 1GB, 2))}}
It'll return something like this:
Name TotalSizeGB
---- -----------
testvol 250
The TotalSize is reported in bytes, so I'm just using a calculated property to convert it back to GB for readability's sake.