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.
The simplest thing to do would be to use inch measurements in your CSS code, and hope that the chain of software from your browser to the printer driver to the printer all know about and agree on the printer's resolution.

It would be a little more effort to actually determine the resolution of the printer, either from its documentation or by counting dots in one inch. Then you could equate one printer dot to one pixel and continue to do your CSS layout measurements in pixels.