如下，需要ffmpeg并且设定路径，它自己会尝试遍历并生成压缩尺寸后的视频：

$ErrorActionPreference = 'Continue'
$ProgressPreference = 'SilentlyContinue'

$folders = @(
    'F:\电影\神探狄仁杰1',
    'F:\电影\神探狄仁杰2',
    'F:\电影\神探狄仁杰3',
    'F:\电影\神探狄仁杰4'
)
$exts = '*.mp4','*.mkv','*.avi','*.mov','*.wmv','*.webm','*.ts','*.m2ts'

Write-Host '===== Scanning =====' -ForegroundColor Cyan
$all = @()
foreach ($d in $folders) {
    if (-not (Test-Path -LiteralPath $d)) { continue }
    foreach ($e in $exts) {
        $found = Get-ChildItem -LiteralPath $d -Filter $e -Recurse -ErrorAction SilentlyContinue
        foreach ($f in $found) {
            if ($f.FullName -notmatch '[\\/]compressed[\\/]') {
                $all += $f
            }
        }
    }
}
Write-Host ('Found ' + $all.Count + ' files') -ForegroundColor Cyan

# Phase 1: probe
Write-Host ''
Write-Host '===== Probing =====' -ForegroundColor Cyan
$todo = [System.Collections.ArrayList]::new()
$skipList = [System.Collections.ArrayList]::new()
$badList = [System.Collections.ArrayList]::new()

for ($i = 0; $i -lt $all.Count; $i++) {
    $f = $all[$i]
    Write-Host ('[' + ($i+1) + '/' + $all.Count + '] ' + $f.Name) -ForegroundColor Gray

    $tmpFile = [System.IO.Path]::GetTempFileName()
    $probeArgs = '-v','error','-select_streams','v:0','-show_entries','stream=width,height','-of','csv=s=x:p=0',$f.FullName
    $p = Start-Process -FilePath ffprobe -ArgumentList $probeArgs -NoNewWindow -Wait -PassThru -RedirectStandardOutput $tmpFile
    $info = (Get-Content -LiteralPath $tmpFile -Raw).Trim()
    Remove-Item -LiteralPath $tmpFile -Force -ErrorAction SilentlyContinue

    if ($p.ExitCode -ne 0 -or [string]::IsNullOrEmpty($info)) {
        Write-Host '  FAIL: ffprobe' -ForegroundColor Red
        [void]$badList.Add($f.Name)
        continue
    }

    $parts = $info -split 'x'
    $w = 0; $h = 0
    try { $w = [int]($parts[0].Trim()); $h = [int]($parts[1].Trim()) } catch {}
    if ($w -eq 0 -or $h -eq 0) {
        Write-Host ('  FAIL: bad res ' + $info) -ForegroundColor Red
        [void]$badList.Add($f.Name)
        continue
    }

    if ($w -le 3200 -and $h -le 1440) {
        $resStr = $w.ToString() + 'x' + $h.ToString()
        Write-Host ('  SKIP: ' + $resStr) -ForegroundColor DarkGray
        [void]$skipList.Add($f.Name + ' (' + $resStr + ')')
    } else {
        $resStr = $w.ToString() + 'x' + $h.ToString()
        Write-Host ('  QUEUE: ' + $resStr + ' -> 3200x1440') -ForegroundColor Yellow
        [void]$todo.Add([PSCustomObject]@{File=$f;W=$w;H=$h})
    }
}

Write-Host ''
Write-Host ('Result: todo=' + $todo.Count + ' skip=' + $skipList.Count + ' fail=' + $badList.Count) -ForegroundColor White

if ($todo.Count -eq 0) {
    Write-Host 'Nothing to process.' -ForegroundColor Yellow
    exit 0
}

# Phase 2: encode
Write-Host ''
Write-Host ('===== Encoding: ' + $todo.Count + ' videos =====') -ForegroundColor Cyan
$ok = 0
$ng = 0
$dup = 0
$sw = [System.Diagnostics.Stopwatch]::StartNew()

for ($i = 0; $i -lt $todo.Count; $i++) {
    $item = $todo[$i]
    $f = $item.File
    $w = $item.W
    $h = $item.H
    $bn = [System.IO.Path]::GetFileNameWithoutExtension($f.Name)

    $root = ''
    foreach ($d in $folders) {
        $dn = $d.Replace('/','\')
        if ($f.FullName.StartsWith($dn, [StringComparison]::OrdinalIgnoreCase)) {
            $root = $d
            break
        }
    }
    if ($root -eq '') { $root = $f.DirectoryName }

    $outDir = Join-Path $root 'compressed'
    $outPath = Join-Path $outDir ($bn + '.mkv')

    Write-Host ('[' + ($i+1) + '/' + $todo.Count + '] ' + $f.Name + ' (' + $w + 'x' + $h + ')') -ForegroundColor Green

    if (-not (Test-Path -LiteralPath $outDir)) {
        New-Item -ItemType Directory -Path $outDir -Force | Out-Null
    }

    if ((Test-Path -LiteralPath $outPath) -and (Get-Item -LiteralPath $outPath).Length -gt 10240) {
        Write-Host '  SKIP: output exists' -ForegroundColor DarkGray
        $dup++
        continue
    }

    Write-Host '  Encoding...' -ForegroundColor DarkYellow

    $ffArgs = @(
        '-i', $f.FullName,
        '-map', '0',
        '-c:v', 'hevc_amf',
        '-rc', 'cqp',
        '-qp_i', '22',
        '-qp_p', '24',
        '-qp_b', '24',
        '-quality', 'quality',
        '-vf', 'scale=3200:1440:force_original_aspect_ratio=decrease:flags=lanczos,format=yuv420p',
        '-c:a', 'copy',
        '-c:s', 'copy',
        '-y',
        $outPath
    )

    $fp = Start-Process -FilePath ffmpeg -ArgumentList $ffArgs -NoNewWindow -Wait -PassThru

    if ($fp.ExitCode -eq 0 -and (Test-Path -LiteralPath $outPath) -and (Get-Item -LiteralPath $outPath).Length -gt 10240) {
        $inMB = [math]::Round($f.Length / 1MB, 1)
        $outMB = [math]::Round((Get-Item -LiteralPath $outPath).Length / 1MB, 1)
        Write-Host ('  DONE: ' + $bn + '.mkv  in=' + $inMB + 'MB out=' + $outMB + 'MB') -ForegroundColor Green
        $ok++
    } else {
        Write-Host ('  FAIL: code=' + $fp.ExitCode) -ForegroundColor Red
        Remove-Item -LiteralPath $outPath -Force -ErrorAction SilentlyContinue
        $ng++
    }
    Write-Host ''
}

$sw.Stop()

Write-Host '========================================' -ForegroundColor Cyan
Write-Host '  Complete!' -ForegroundColor Cyan
Write-Host '========================================' -ForegroundColor Cyan
Write-Host ('  Compressed : ' + $ok) -ForegroundColor Green
Write-Host ('  Skipped    : ' + ($skipList.Count + $dup)) -ForegroundColor Yellow
Write-Host ('  Failed     : ' + $ng) -ForegroundColor Red
$t = [math]::Floor($sw.Elapsed.TotalMinutes).ToString() + 'm ' + $sw.Elapsed.Seconds.ToString() + 's'
Write-Host ('  Time       : ' + $t) -ForegroundColor White
Write-Host '  Output     : F:\电影\神探狄仁杰1\compressed\' -ForegroundColor Cyan
Write-Host '========================================' -ForegroundColor Cyan