SharePoint Online: Get File Size using PowerShell

Requirement: Get file size in SharePoint Online using PowerShell

How to Get the File Size in SharePoint Online?
To get the size of a file, you can follow these approaches: 

• Edit the list view of the document library and include "File Size" column to it
• Go to "Storage Metrics" page and you can get size of the files.
• You can open the library in explorer view and get the file size from file's properties. 
• You can open the site in SharePoint Designer to get size of the files.
  

PowerShell to Get File Size in SharePoint Online:
To get file size in SharePoint Online, use this PowerShell script

#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
 
Function Get-SPOFileSize($SiteURL,$FileRelativeURL)
{
    #Setup Credentials to connect
    $Cred = Get-Credential
    $Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.UserName,$Cred.Password)
 
    Try {
        #Setup the context
        $Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
        $Ctx.Credentials = $Credentials
 
        #Get the File
        $File = $Ctx.Web.GetFileByServerRelativeUrl($FileRelativeURL)
        $Ctx.Load($File)
        $Ctx.ExecuteQuery()
        $FileSize = [Math]::Round($File.Length/1KB,2)

        #Get File Size
        Write-host "File Size (KB):" $FileSize
    }
    Catch {
        write-host -f Red "Error:" $_.Exception.Message
    }
}
 
#Call the Function to get file size with Site URL and File URL
Get-SPOFileSize -SiteURL "https://crescent.sharepoint.com/sites/marketing" -FileRelativeURL "/sites/marketing/Branding/Technical Design.docx"

To get the total size of a file with its versions, use:

#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"

#Parameters
$SiteURL = "https://crescent.sharepoint.com/sites/marketing" 
$FileRelativeURL =  "/sites/marketing/Branding/Technical Design.docx"
 
#Setup Credentials to connect
$Cred = Get-Credential
$Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.UserName,$Cred.Password)
 
Try {
    #Setup the context
    $Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
    $Ctx.Credentials = $Credentials
 
    #Get the File
    $File = $Ctx.Web.GetFileByServerRelativeUrl($FileRelativeURL)
    $Ctx.Load($File)
    $Ctx.Load($File.ListItemAllFields)
    $Ctx.ExecuteQuery()
    $TotalFileSize = [Math]::Round($File.ListItemAllFields.FieldValues.SMTotalSize.LookupId/1KB,2)

    #Get File Size
    Write-host "Total File Size (KB):" $TotalFileSize
}
Catch {
    write-host -f Red "Error:" $_.Exception.Message
}

SharePoint Online: Get File Size using PnP PowerShell
To get File Size using PnP PowerShell

#Parameters
$SiteURL = "https://crescent.sharepoint.com/sites/marketing" 
$FileRelativeURL =  "/sites/marketing/Branding/Technical Design.docx"
 
#Connect to PnP Online
Connect-PnPOnline -Url $SiteURL -UseWebLogin
 
#Get the File as List Item
$File = Get-PnPFile -Url $FileRelativeURL -AsListItem
 
#Get File Size
Write-host "File Size:"$File["File_x0020_Size"]

If you want to analyze file size for all documents, use the below PowerShell script.

Get File Size for all documents in a SharePoint Online Document Library
Let's get size of the each file in a document library and export the data to a CSV file using PowerShell:

#Parameters
$SiteURL = "https://crescent.sharepoint.com/sites/Marketing"
$ListName= "Branding"
$ReportOutput = "C:\Temp\FileSizeRpt.csv"
  
#Connect to SharePoint Online site
Connect-PnPOnline $SiteURL -UseWebLogin

#Array to store results
$Results = @()
  
#Get all Items from the document library
$List  = Get-PnPList -Identity $ListName
$ListItems = Get-PnPListItem -List $ListName -PageSize 500 | Where {$_.FileSystemObjectType -eq "File"}
Write-host "Total Number of Items in the List:"$List.ItemCount

$ItemCounter = 0 
#Iterate through each item
Foreach ($Item in $ListItems)
{
        $Results += New-Object PSObject -Property ([ordered]@{
            FileName          = $Item.FieldValues.FileLeafRef
            RelativeURL       = $Item.FieldValues.FileRef
            FileSize          = $Item.FieldValues.File_x0020_Size
            TotalFileSize     = $Item.FieldValues.SMTotalSize.LookupId
        })
    $ItemCounter++
    Write-Progress -PercentComplete ($ItemCounter / ($List.ItemCount) * 100) -Activity "Processing Items $ItemCounter of $($List.ItemCount)" -Status "Getting data from Item '$($Item['FileLeafRef'])"
}
 
#Export the results to CSV
$Results | Export-Csv -Path $ReportOutput -NoTypeInformation
Write-host "File Size Report Exported to CSV Successfully!"

Here is my another post on adding file size column to SharePoint Online document library list view: How to add file size column in SharePoint Online?