How can I control the width of a label tag?

Using CSS, of course...

label { display: block; width: 100px; }

The width attribute is deprecated, and CSS should always be used to control these kinds of presentational styles.

Using the inline-block is better because it doesn't force the remaining elements and/or controls to be drawn in a new line.

label {
  width:200px;
  display: inline-block;
}