Although the WYSIWYG Editor allows you to 're-size' the image dimensions we recommend you re-size your images prior to uploading to the server.
This has two main benefits:
A: The image used in the site is small and pages will load faster
B: The upload to the server will be a lot quicker as well.
Most of the Photo Editing Software has functionality to re-size your images, however not many have Batch Functionality.
To batch process your images we recommend a very easy to use piece of software called: Mihov Image Re-sizer.
http://www.mihov.com/eng/ir.htmlThis will enable you to re-size to any size you prefer and also set the quality of the images thereby setting the file size of the image.
An image of 250 pixels in width should be no more then around 15Kb to 20Kb.