It requires some basic CSS coding. CSS is incredibly easy, and I think I have the right to make that statement, because I suck at coding in everything else.
Basically, you give the excerpt box a max-height, and you give the excerpt text an overflow:hidden .
If you need help, e-mail me at email@example.com
If you don't know anything about CSS, I suggest you start by using the Firefox web browser and installing the Firebug plugin from the official Firefox plugins page. Then go to your blog and right-click anything on your blog and click "Inspect Element". Take a good look at Firebug for a while, and screw around with the CSS code within the plugin so you can get an idea for how CSS works.
You can then ask me for extra help if you want. Good luck!