Hi,
To adjust the class selector region-content for any one of these regions you can do it like so...
#page-content #region-main .region-content { line-height: 1.4}
#page-content #region-pre .region-content { line-height: 1.4}
#page-content #region-post .region-content { line-height: 1.4}
Line height is set in #page in the BASE theme, it is also set by default in the body tag automatically from the YUI CSS library in moodle/lib/yui/... but don't go there, as there be dragons!
Dependng on your theme you can add this via the Custom CSS Settings page (if there is one) or add this directly to your theme's core.css or whatever the main CSS file is called.
Mary