This is about 400 to 480 pixels. If you place your text inside a centered table set to that, you'll optimize your message for web readability. Don't use all caps . All caps are difficult to read because our eyes are trained to recognize word shapes, and all caps slows down our shape recognition. It's also considered rude.
White space means open areas of the page without graphics or text.
Lack of white space is visual clutter. It's confusing to visitors. Don't center text . You can center headlines, but a page full of centered text is the first sign of a newbie. You may have to reword a line here and there to keep some sentences from becoming too strung out, but it's worth it.