Label width in horizontal forms

In a horizontal form, each label and its corresponding form item are in one line. To make these labels aligned properly, they must be of equal width. Then what width should we choose? There're at least three options:

1. Specify a proportion to the width of the line.
2. Specify a fixed width.
3. Let the width be determined by the longest label automatically.

I like the third option best as we don't need to specify anything. Also, we wouldn't bother with adjusting anything(like the proportion or the fixed width) to ensure label text not overflowed when we add another form line.

What do you think?

Edit: If each line of a form contains only one label and one form control and the label and the form control are arranged horizontally, I call it a horizontal form. Here's an example.