Sanjeev,
I am not an expert on this either: but I found some information http://www.simplemachines.org/community/index.php?topic=291694.0 that suggests that publishing the full feed COULD put more processing load on the server. I don't think this will be a problem as this is a relatively small community of readers on this board (less than 2000 if I recall correctly). For comparison, some forums would likely have 10 or 20 times as many users. Furthermore, I bet a lot of us will continue to access the forum via the website so that we can use other useful features such as the "Show unread posts since last visit" link at the top.
There is some more information about the forum software's RSS settings here:
http://www.simplemachines.org/community/index.php?topic=25009.0
I believe this setting should only control the max length of each post published in the RSS feed. It should be safe to experiment with different values. Keep in mind, if your RSS feed reader caches the content you may need to click refresh to notice the changes to the post length.