Question: First of all, I am talking about HTML.

My client asked me to make a form that he is going to print with a thermo printer. He told me that the thermo printer uses 4x6'' papers. What width and height should I use on CSS?

Answer: 
You can use inch in your (print) CSS:

```
#foo { width: 5in; height: 3in; }

```

See [length units](http://www.w3.org/TR/CSS21/syndata.html#length-units) in CSS.
If you *must* use HTML, use physical measurements as pointed out in the other questions.

In my opinion though, printing labels is not wise to do from HTML. HTML is not built for the level of correct printing you need for this task. Consider generating a PDF using a library like [fpdf](http://www.fpdf.org/), it's easy and reliable.