Windows 7 (Aero) does not offer an option to easily change the taskbar font size.
I was using the Medium Text size of 125%.
(Set via Control Panel\Appearance and Personalization\Display)
The problem I found was that many programs, sad to say, Microsoft's included, don't scale or position properly at other than 100%. I've reset the DPI to 100%, and have been able to customize most of my windows settings for easy reading, except the task bar font size
You can not change the appearance of the taskbar in that way, The only alternative I see is not using the default Explorer shell (taskbar & desktop). Windows has the ability to use alternative shells:
The taskbar font size can be changed by changing the font size of the Active Title Bar setting in the Windows Color and Appearance window.
Active Title Bar
Windows Color and Appearance
That window is reachable via Control Panel -> Appearance and Personalization -> Personalization -> Window color -> Advanced appearance settings.
But there is a gotcha : it works in the Classic theme, but not in Aero.
Otherwise, I can see no solution for Aero, except by replacing the taskbar. See :
Best Free Program Launcher (including comments)
The Best Application Launchers and Docks for Organizing Your Desktop
As you say that the taskbar font is not the only problem left unsolved,
I don't think that a complete solution is possible under Aero in Windows 7.
You might consider seeing whether Windows 8, and especially Windows 8.1 (named Blue),
can do better with its new algorithms for seamless DPI scaling.
Here are some links about Windows 8 and 8.1 :
Building Windows 8 : Scaling to different screens
In Blue: Automatic Desktop Display Scaling
I don't have a concrete answer for you, but I can tell you where I would start. I warn you that this line of inquiry is not for the faint of heart.
The style information for the aero theme under Windows 7 is located in a resource-laden DLL called shellstyle.dll, in C:\Windows\Resources\Themes\Aero\Shell\NormalColor. The file's resources can be viewed with a tool called Resource Hacker, available here. Upon opening the file, you should browse to /UIFILE/1/1033 - and you'll find the XML definitions of the styles used by Aero, as shown below. This should include most of the fonts, sizes, and icons used throughout explorer. CSS Definitions are in /23/CPWEBVM.CSS.
I've successfully edited this resource before to remove a number of UI elements I didn't like in explorer, such as the Command Bar. If the font for taskbar entries is defined anywhere at all, it will be in this file. It's also possible that this font inherits from some global value and can be overridden in this file - there's actually a good deal of flexibility, but you need to know what you're doing.
If you want to make changes, back up the original file, take ownership, and replace it. Then you'll need to manually restart explorer.exe to see the effects. If you have any doubts whatsoever about editing this file, I would leave it well alone and learn to live with the existing fonts in the taskbar buttons.
Not an great selection,but may be better
Start Menu>right click on >properties>Taskbar tab>Taskbar appearance>use small icons
You could change the DPI. It's not the same thing as the changing resolution. Which will also change the size.
Check if you can configure the right option by editing the registry.
Win+R → regedit
Navigate to HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics, here you will find various values that you can set.
It is explained with screenshots here: http://www.wikihow.com/Customize-the-Size-of-the-Windows-7-Taskbar-Icons.
Check the Registry section.
Is there a way to change the size of the Windows 7 Taskbar? Internal or with the help of outside programs, both welcome. Something like the MacOS X Doc Zooming effect would be OK/nice, too.
I'm essentially looking for a way to shrink it, because my laptop does not have a big screen so every pixel is valueable.
Right-click on the Taskbar, uncheck Lock the taskbar.
You can increase its height by grabbing its side and dragging it up.
For the zooming effect, not possible by default, unless you replace the taskbar with an alternative dock. There are some suggestions here. LifeHacker has also a list of application docks.
OK, I did some digging...
Don't know whether this is relevant for you, but I could not shrink the width as narrow as I wanted until I removed some 3rd party tools from the taskbar (Thinkpad stuff: AccessConnections etc). Once I had removed them, I could drag the edge much thinner.
Shark's solution kind of worked for me, but make sure you also use small icons for your taskbar.