AFAIK, TinyMCE does not have a setting for limiting file size, only attributes like base upload path, file types and file names (see TinyMCE doc)
Could it be you're confusing it with the global
maxAllowedContentLength attributes in web.config (IIS) ?
You would need to create a custom plugin for that.
If you're already using ImageResizer (https://imageresizing.net/), I guess the easiest thing would be to create a display template for XhtmlString property (Views / Shared / DisplayTemplate / XhtmlString.cshtml) and append width/height to images.
I would go with the ImageResizer track too. Let them upload but automatically scale image to a fixed max width/height. You need a license though for ImageResizer to get the disc cache working but it's a negligable cost.
I wrote a blog post on this one: https://www.dcaric.com/blog/resizing-images-in-episerver
When editors uploads though the richtext editor, I want tochange the upload limit the size of files, to limit the size of images.
There is a setting but it seemse i can not find anywhere. is there anywhere where this can be configeruared?
Or do i have to buildt a new pllugin for the editor?