Windows 7+ Taskbar Thumbnail Tweaker is a freeware from door2windows which lets you easily change the taskbar thumbnails' size, margin, spacing, max thumbnails and text height in Windows 7, Windows 8 and Windows 8.1.
How to use:
Run the tool and customize the settings as you like. After you are done, click Change. The Windows Explorer will now restart automatically and you will be able to see the new settings which you chose. If you want to revert to the original settings click Restore.