With this powershell command you will be to find the size of the media library item.
Import-Function -Name ConvertTo-Xlsx
$pdfItems= gci -path "/sitecore" -Recurse |
Select-Object -Property Name, ID, ItemPath, TemplateName, Size |
Sort-Object -Property Name
[byte[]]$outobject = $pdfItems | ConvertTo-Xlsx
Out-Download -Name "Media_Size.xlsx" -InputObject $outobject