I'm trying to change the preview iframe size in the Image plug-in. 
I've been looking through the javascript for the plug-in(plugins/image/dialogs/image.js), have been able to increase the size of the entire pop-up for the plug-in, but haven't had success with the preview window where the image comes up among text.
I have tried:
changing the second width on line 360 if you beautify(http://www.jsbeautifier.org) the javascript:
widths: ['140px', '240px'],
Re: Change preview iframe size in Image plug-in
YO FIGURED IT OUT MYSELF, after spending a couple days researching how to write plug-ins for this undocumented beast. Look at this faaaat preview box:
Here are the steps to increase the preview iframe size:
1. Open ckeditor/plugins/image/dialogs/image.js for editing, which is the only file necessary to edit.
2. Beautify (http://jsbeautifier.org/) the file for the line references below.
3. Increase the minWidth and minHeight on lines 148-149. I use 620 and 510 respectively.
4. Increase the second width parameter on line 361. I use 400.
5. Increase the height on line 668. I use 400.
6. On line 672, add a style definition to the div with id 'ImagePreviewBox' to set the width and height of the actual preview box. This is my tag: <div id="ImagePreviewBox" style="width:450px;height:400px;">
7. Repack the javascript file (http://dean.edwards.name/packer/) and save it.
THAT'S IT YO.