Hello Markus,
Sorry for late reply to this,
Yes, it is possible to resize your blog images.
May i ask you when you click save that moment, do you have click the "Clean image" cache? If you doesn't click it "Clean image" button, when you refresh it your page, that blog image size still loading your before that setting size. If you follow this my step, still can't make it, please refer back to us and provide with your Joomla backend and FTP access so we can help you check on this issues.
Please advise.