You can use the following code:
$resourceGroup = "bloblisttestrg"
$storageAccountName = "contosobloblisttest"
$containerName = "listtestblobs"
# get a reference to the storage account and the context
$storageAccount = Get-AzStorageAccount `
  -ResourceGroupName $resourceGroup `
  -Name $storageAccountName
$ctx = $storageAccount.Context 
# get a list of all of the blobs in the container 
$listOfBLobs = Get-AzStorageBlob -Container $ContainerName -Context $ctx 
# zero out our total
$length = 0
# this loops through the list of blobs and retrieves the length for each blob
#   and adds it to the total
$listOfBlobs | ForEach-Object {$length = $length + $_.Length}
# output the blobs and their sizes and the total 
Write-Host "List of Blobs and their size (length)"
Write-Host " " 
$listOfBlobs | select Name, Length
Write-Host " "
Write-Host "Total Length = " $length
Hope this helps!
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