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.
You should use `6 pc` (i.e. `72 pt`) width in your css for the `@media: print` rules. The height... Depending on the content, the number of pages will vary.