Page Specific Font Size

Hi After a long time I’m starting to get into website development again. I’m able to do things much faster this time round because of my previous endeavor (which was a catastrophic failure of a website). But I’m still a noob to all the code being used whether its CSS, HTML, PHP or what have you. My style sheet contains the following code which determines font size on ALL my pages and posts:

body {font: 70%/1.3 ‘trebuchet ms’, verdana, ‘Lucida Grande’, arial, sans-serif; text-align:center; color:#111;}

My question is, how can I rework the code so that I can have different sizes for different posts/pages and not one universal size?

Thanks in advance, webtycoon.