[css] How do I set the size of an HTML text box?

Your textbox code:

<input type="text" class="textboxclass" />

Your CSS code:

input[type="text"] {
height: 10px;
width: 80px;
}

or

.textboxclass {
height: 10px;
width: 80px;
}

So, first you select your element with attributes (look at first example) or classes(look last example). Later, you assign height and width values to your element.