Font size and header/page name

This may sound like a dumb question but how do I change the font size for the content of a page? I have looked everywhere I can think of!

Not only that but as you can see here:
http://www.loopydogs.com if you go to chewing page, there is a huge gap at the top of the content that I am having trouble getting rid of. Also I want the page to be called chewing in the side bar, but I didn’t really want chewing to come up as the title, i wanted that to be ‘How to stop puppy chewing everything!’ But if I change the page title to this then it will show up as that on the side bar.

Any help would be appreciated 🙂