Displaying two rows is quite difficult to achieve as it has to do with how big the font is: what would be one row on one pc could be four rows on another. Limiting the length of the announcement would be a better requirement. You could always try to play with the settings a bit to get a string which would in general be about two rows long, that's as far as you can get.
From your question I assume you mean a content aggregation. If you're using the Content Query Web Part you could easily do that in XSLT using the substring function, like:
<xsl:value-of select="substring(@YourField, 0, 100)"/>
Which would return the first 100 chars of your announcement.
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How to limit number of rows in Announcement body
