W7 Taskbar Tweaker is a freeware portable app that lets you tweak the Taskbar Buttons & Thumbnail Preview sizes in Windows 7.
The developer created this application after realizing that the buttons on the taskbar take up too much space unless you combine them, which some may not like to do.

The best way to do it would be to use Small Icons and set button size to 30 or more. If you use the Large Icons, then set the value to 42 or more. Due to different settings on different PC's, choose and adjust settings that work with your screensize.

This tweak will not work if icons are set to Always Combine.

You can also change the size of the Taskbar Thumbnail Previews. While the default is 96, you can choose a value between 96 & 512, using the slider, easily.

The license of this software is Freeware, you can free download and free use this system optimizer software.