Even if you have file size validation implemented, you will get PostTooLargeException if you're trying to upload file larger than size specified in your php.ini config file. This gist shows how you can handle PostTooLargeException in Laravel.

Handler.php

<?php

// App\Exceptions\Handler::render()
public function render($request, Exception $exception)
{
    if ($exception instanceof PostTooLargeException) 
    {
      // todo: return response or do whatever you need to do
      return  response()->json(['message' => 'File is too large.'], 422);
    }
    
    return parent::render($request, $exception);
}