GetFiles does not return file names, rather it returns either relative or absolute filepaths, depending on the form of the directory specification given. For example, assume the calling process has a working directory of "C:\TEMP" and that "C:\TEMP" contains two files: "A.Dat" and "B.Dat".

GetFiles("C:\\TEMP") will return:

"C:\TEMP\A.Dat"

"C:\TEMP\B.Dat"

GetFiles("C:") will return:

"C:A.Dat"

"C:B.Dat"

GetFiles(".") will return:

".\A.Dat"

".\B.Dat"

If you want just file names not paths you can use Path.GetFileName.

More on Path.GetFileName here
http://msdn.microsoft.com/en-us/library/system.io.path.getfilename.aspx

<#
.SYNOPSIS
    This script displays the size of a folder
.DESCRIPTION
    This script is a rewrite of an MSDN sample. It 
    uses System.IO namespace to determine the size
    of a folder and its subfolders, then displays it.
    Note that the Get-DirSize function is recursive! 
.NOTES
    File Name  : Get-FolderSize.ps1
 Author     : Thomas Lee - tfl@psp.co.uk
 Requires   : PowerShell V2
.LINK
    This script posted to:
     http://pshscripts.blogspot.com/2009/10/get-foldersizeps1.html
    MSDN Sample posted at:
     http://msdn.microsoft.com/en-us/library/system.io.directory.aspx
.EXAMPLE
    PSH [C:\foo]: .Get-FolderSize c:\foo
    The size of C:\Foo and its subdirectories is 17,577,318 bytes
#>

param (
$folder = "C:\Foo"
)

##
# Helper Function Get-DirSize
##
function Get-DirSize {
param ([system.IO.DirectoryInfo] $d)
$Size = 0;    

# Add file sizes.
$fis = $d.GetFiles();
foreach ($fi in $fis){      
         $size += $fi.Length;    
        }
# Add subdirectory sizes recursively.
$dis = $d.GetDirectories()
foreach ($di in $dis) {
     $Size += Get-DirSize($di)   
     }
 return $Size 
}

## End of Get-DirSize helper function 

##
# Start of Script
$d = New-Object system.IO.DirectoryInfo $folder
$size= Get-Dirsize $d
"The size of {0} and its subdirectories is {1} bytes" -f $d,$size.ToString("###,###")