How do I customize the width of the icons in the Windows 7 taskbar?

"You can decrease icon size altogether via small icons...

Right click a blank space on the Taskbar
Select Properties from the context menu
In the ""Task Bar and Start Menu Properties"" dialog, select the ""Taskbar"" tab
Check the ""Use small icons"" checkbox

or modify the registry to target the width only:

Go to HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics\MinWidth
If it is not there, right click on an empty white space and select New -> String Value. Name this entry MinWidth.
If you want only the buttons to show then 38 will suffice if you're using ""small buttons"", else 52 for large buttons. If you set it below 38, the buttons won't work properly.
Log off or restart to see changes."