The above answers assume that you send the full text to the browser, then let it only display a certain amount of it by clipping it vertically. This is actually a good idea, as truncating a text afer a certain amount of characters is actually not as straight-forward as it seems.
In an early project, I had a long list of truncated texts and didn't want to send them all to the browser in full length. The important thing to keep in mind here is if your text may contain control or escape characters (e.g. HTML, BBCode, HTML-Entities, etc) you need to take special care about them.
I ended up writing a small HTML-tag parser to not deliver HTML tags which were cut in half, and to add end-tags to e.g. bold, italic, etc, to not screw up the rest of the screen layout.
Additionally, it's usually not what you want - i.e. you won't get two lines worth of text for different screen widths or when having line break characters in your text.