This sample shows different options to control what to do with big images with the SimpleUpload plugin.
You can read about other features here
This editor uses the main settings, there are no restrictions on the images
This editor sends the image "as-is" to the server, and it's there where the image is resized with the CKFinder configuration
CKFinder is configured to allow a maximum of 400x300 px.
The editor uses the simpleuploads_maximumDimensions settings to reject any image that it's bigger than 400x300
Any image here is automatically resized before it's uploaded to the server.
A listener is set on the editor to call the "scaleImage" function that will take care of resizing the image on the browser
Check the source of this page to find out all the details
Keep in mind that this is just a quick sample and so the quality of the resizing is restricted by the browser and you can easily find out that it isn't quite good. For proper implementation of this approach, you should get the ImageCrop Plugin that besides resizing the photos with different sizes, allows the user to select only the important part.