I think this will size your images the way you want.
First, you will need to add a rule to your style.css to resize all the images that you assign to the 'fullwidth' class. Add this to the end of style.css:
Next, you will need to add the 'fullwidth' class to each image that you need to expand. You can do this by installing the Ultimate TinyMCE plugin which adds additional capabilities to the editor.
You will need to configure the plugin by going to Admin->Settings->Ultimate TinyMCE, clicking 'Default Settings', checking 'Please confirm before proceeding' and clicking 'Load Defaults'.
Now, edit a post and click on an image. You should see two small icons pop up at the top of the image. Click on the one that looks like a picture to open the image edit dialog.
Click 'Advanced Settings' and add 'fullwidth' to the box for 'CSS Class'. Be sure there is a space between that and any other classes that are already there.
Scroll down and click 'Update' to save the changes.
Update the post and that should do it.