Here's the result
D:\ 14.754878997802734375 GB,C:\ 49.656246185302734375 GB
Based on your suggestion i was able to adapt my first idea. This one output exactly what i want
Get-VM "MyVM" | Select @{N='Path';E={((get-vm $_).extensiondata.Guest.Disk | sort $_.Diskpath | %{" $($_.diskPath) $([math]::Round($_.Capacity /1024MB)) GB"}) -join ','}}
Output :
C:\ 50GB, D:\ 15GB
Thx a lot
Raf