I've had the same problem and as I like to make followers (dishy guys) and like to dress them just right it is a real pain to have this problem as it takes hours more of your valuable time!
After uninstalling and reinstalling deleting local content and verifying cache all of which made no difference I have just solved the width issue by editing the SkyrimEditorPrefs ini file found in the "Steamapps/common/skyrim" folder.
To edit preview window size
1) Make a copy of the SkyrimEditorPrefs file and call it SkyrimEditorPrefsCOPY, just in case you accidentally make a hash of it (it happens), save it in the same folder so it's easy to find if you need it!
2) Open the original file and look under "General" starting at the top, down about 6-8 inches from top of list.
Here is a section of the list with the bit you want to find, separated out in green;
Copy the numbers that you see under the iPreview section above and change yours to these values. Save the file.
The original (shown below in red) gives these figures, make sure yours is not in reverse order as mine was;
As you can see the iPreviewW ( width) is originally set to 158 which is why the box was so thin. I don't know why this is but assume someone at Bethesda just screwed up, it's the only box that can't be resized by dragging!
Mine works fine now after years of struggling! Hope it helps you.