Select-Object
will be your friend here:
foreach ($c in (get-content .\Servers.txt))
{ Get-Childitem \\$c\y$\mydirectory | select-object @{l="Name" ; e = {$c}},Lastwritetime,@{l="Size" ; e={(Get-childitem -recurse | measure-object length -sum).sum}} }
But you could also do yourself a favor and add a function like get-foldersize to your profile or to a standard tools module.
http://gallery.technet.microsoft.com/Get-FolderSize-b3d317f5