$upperBound = 100MB # calculated by Powershell
$ext = "log"
$rootName = "log_"
$destination = "F:\Apps\Visualcron\log\"

$reader = new-object System.IO.StreamReader("F:\Apps\FileZilla Server\Logs\fzs-2022-02-09.log")
$count = 1
$fileName = "{0}{1}{2}.{3}" -f ($destination, $rootName, $count, $ext)
while(($line = $reader.ReadLine()) -ne $null)
{
    Add-Content -path $fileName -value $line
    if((Get-ChildItem -path $fileName).Length -ge $upperBound)
    {
        ++$count
        $fileName = "{0}{1}{2}.{3}" -f ($destination,$rootName, $count, $ext)
    }
}
$reader.Close()