How to increase font size of sidebars?

The font size is probably hardcoded in your context theme, using the default theme should fix this.

I think for all other tabs in the sidebar you will have to change to the KDE font settings -> install the package 'systemsettings' and start that application (e.g. via a terminal using the exactly same name).

Mr. Sitter, please explain these suggestions for me (I am something of a newbie):

>I think for all other tabs in the sidebar you will have to change to the KDE font settings -> install the package 'systemsettings' <

1. Where do I get the package 'systemsettings'? (I looked in Hardy's Add/Remove - it didn't come up in the search bar.)

>and start that application (e.g. via a terminal using the exactly same name).<

2. What application are you telling me to start?

3. Do you mean for me to put something like "sudo [exact same name of application]" in a ternimal window?

I'll go with the fast method. Alt+f2 provides some kind of quick starter.
0) If you have a standard Kubuntu installation you can jump right to 6)
1) Press Alt+f2
2) enter 'konsole' if you are on Kubuntu or 'gnome-terminal' if you are on Ubuntu (without '')
3) a terminal window should open
4) enter 'sudo apt-get install systemsettings kcontrol' (without '')
5) it should download the appropriate packages
6) close the terminal and press Alt+f2 again
7) enter 'systemsettings' (without '')

Some additional information if you want to understand the above :)...
1) there are various possibilities to start applications on Linux, one is via the menu, another one would be this Alt+f2 starter and in fact you can start applications from a terminal
2) konsole and gnome-terminal are so-called terminal emulators, they are kind of fake-terminals (you can't really have a terminal and a graphical desktop at the same time, so these terminal emulators are one useful piece of software for advanced users), to get to a real one you can press ctrl+alt+f1 (you get back to your desktop via ctrl+alt+f7). There are tons of these applications beside konsole and gnome-terminal, e.g. xterm or eterm.
4) sudo gives you root access for command after the sudo, which means sudo konsole will open konsole as root. This is a very important tool because on Linux a user usually only can change data in his home directory. if you want to install applications or change anything outside your home directory you need to use sudo (there are also graphical versions like kdesudo/gksu these are the preferred way of starting graphical applications, so one would really use kdesudo konsole ;-)
4.1) apt-get is a very basic, but powerful, tool to manage software on Ubuntu/Kubuntu/Debian and a lot of other Linux distributions. It takes care that all necessary software is installed and up-to-date. Add/Remove, Synaptic or Adept would be graphical versions of apt-get
4.2) the last part tells apt-get which application to install, in this case systemsettings
7) instead of alt+f2 you could (as mentioned above) just use the menu or just enter 'systemsettings' in the terminal

systemsettings is not available in 8.10 :/