It depends on the theme that you're using.
As Patrick Ryan said " You should look into using firebug in firefox or inspect in chrome or developer tools in IE. These will all easily show you what css class you need to target to make these and any other changes."
I personally use Chrome so in order to do that, just right click on the site name. Here's a preview in chrome.
you can use #site-name or #name-and-slogan in this theme. But this will change per theme. The same process can be done if you want to align the text or make it wider.
The great thing about using these development tools is that they allow you to add CSS right on the browser. you'll see that on the right side. Just double click.
If you're using a subtheme of Zen, for example, check the templates in the zen folder to see the page.tpl.php file for the template of the page.
I touch on using templates in Zen subthemes in my slides here (slide 44 onwards): http://www.slideshare.net/japodomingo/theming-with-zen
If you know how to use templates, you can manipulate the display any way you want. :) Here's a brief overview of the templates. https://drupal.org/node/190815
I hope this helps.
