Boxes are just big enough to hold text. It's as wide as the browser window. This is the first box.
This is the second box. Text text text text Text text text text Text text text text Text text text text Text text text text
Text text text text Text text text text Text text text text.
You can make boxes a certain pixel (px) size.
And here's one inside another with a percentage size, with centered text.
Here's a box (five) on the left.
Here's a box (six) in the middle. And using "float", it's next to box five.