KMail's settings window gives you a number of ways to customize KMail to your needs.
Note that basic instruction on how to setup KMail (using Identity and Network) are featured in the Getting Started section.
The Fonts section allows you to change the type and size of the display fonts.
The Colors section allows you to change the color of the text. Recycle colors on deep quoting means that even text that's quoted more than three times will appear in color.
The Layout section lets you toggle the long folder list on and off. The long folder list lists folders all the way down the left side of the screen, which lets you see more folders than in the default view, but it takes away some space for viewing the text of the messages. With Display message sizes selected there will be another column in the Header pane that show the messages' size in bytes. Thread list of message headers will put all the messages in a kind of tree, so the replies to a mail are next to that mail. Prefer plain text to HTML rendering will try to show even HTML mails as plain text.
The Profiles offer several common default settings for colors and font sizes.