Adding the file sizes of Get-ChildItem listing

Use the Measure-Object cmdlet:

$files = Get-ChildItem -Recurse -Include *jpg,*bmp,*png \\server01\folder
$totalSize = ($files | Measure-Object -Sum Length).Sum

To get the size in GB divide the value by 1GB:

$totalSize = ($files | Measure-Object -Sum Length).Sum / 1GB

You could do this as a one liner with something like this.

Get-Childitem -path  "C:\Program Files\Internet Explorer" | Select-Object 
Name, @{Name="KBytes";Expression={ "{0:N0}" -f ($_.Length / 1KB) }}

Name          KBytes
----          ------
en-US         0
images        0
SIGNUP        0
ExtExport.exe 52
hmmapi.dll    53
iediagcmd.exe 500
ieinstal.exe  490
ielowutil.exe 219
IEShims.dll   398
iexplore.exe  805
sqmapi.dll    49