font-size, .hentry, and having different font sizes

Hi all, WordPress newbie here. Working on my test and development site at http://kevinburel.net/wordpress and trying to clean things up a bit before going live. As you see, the site has a menu bar on the left side with links to some content elsewhere in my site. I am working to correct two main problems.

1. The text is, in general, too small. Look at a page like http://kevinburel.net/wordpress/?page_id=21 and you have to squint to read the text. Not a good solution.

2. The line height is not tall enough. Notice how, on that page linked above, text with parts below the line (g, p, q) get truncated. Not very professional-looking.

So I tried putting body { font-size: 13px; line-height: 24px; } in my main stylesheet, but that made the text and horizontal lines in the left hand menu bar too big; they were spilling out of the menu bar.

Someone suggested I use .hentry { } to zero in on the content I want to change (make bigger) while leaving the rest of the text on the site alone. But I don’t know how to use .hentry and the little documentation I’ve found isn’t very good. What goes between the brackets? Is it the HTML classes I want to exempt from the font-size, or is it the .phps that influence the look of posts and pages while not affecting the main menu bar. And, finally, where in the code should this line go?

Thanks so much for helping a newbie out!!