This works great for me using forever forward incremental jobs. If you are using synthetic fulls, or reverse you may get unexpected results as your latest job will show as the full size. 

I took my previous script and modified it so I can look at the previous 2 weeks, and see the size of the backup for each day.  If you change the (-14) you can make this weekly, monthly, daily with ease.

# Location of the Veeam backup folder
$BackupFolderPath = "f:\Backups"

# Start and end dates
$EndDate = Get-Date
$StartDate = $EndDate.AddDays(-14)

# Function to convert bytes to GB
function ConvertTo-GB {
    param (
         Parameter(Mandatory=$true)]
         int64]
        $bytes
    )
    return "{0:N2}" -f ($bytes / 1GB) + " GB"
}

# Go through each day from the start date to the end date
for ($Date = $StartDate; $Date -le $EndDate; $Date = $Date.AddDays(1)) {
    # Date in the format 'yyyy-MM-dd'
    $DateString = $Date.ToString('yyyy-MM-dd')

    # Get all backup files for this date
    $BackupFiles = Get-ChildItem -Path $BackupFolderPath -Recurse -Filter "*$DateString*"

    # If there's no backup file for this date, continue to the next date
    if ($BackupFiles -eq $null) {
        Write-Output "No backup files for the date $DateString were found."
        continue
    }

    # Sum up the sizes of all backup files for this date
    $TotalBackupSizeBytes = 0
    foreach ($BackupFile in $BackupFiles) {
        $TotalBackupSizeBytes += (Get-Item $BackupFile.FullName).Length
    }

    # Convert bytes to GB
    $TotalBackupSizeGB = ConvertTo-GB -bytes $TotalBackupSizeBytes

    # Print the total size of the backup files for this date
    Write-Output "The total size of the backup files for the date $DateString is $TotalBackupSizeGB."
}