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

input[type="text"]
{
    width:200px
}

Just use:

textarea {
    width: 200px;
}

or

input[type="text"] {
    width: 200px;
}

Depending on what you mean by 'textbox'.

Your markup:

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

The CSS:

.resizedTextbox {width: 100px; height: 20px}

Keep in mind that text box size is a "victim" of the W3C box model. What I mean by victim is that the height and width of a text box is the sum of the height/width properties assigned above, in addition to the padding height/width, and the border width. For this reason, your text boxes will be slightly different sizes in different browsers depending on the default padding in different browsers. Although different browsers tend to define different padding to text boxes, most reset style sheets don't tend to include <input /> tags in their reset sheets, so this is something to keep in mind.

You can standardize this by defining your own padding. Here is your CSS with specified padding, so the text box looks the same in all browsers:

.resizedTextbox {width: 100px; height: 20px; padding: 1px}

I added 1 pixel padding because some browsers tend to make the text box look too crammed if the padding is 0px. Depending on your design, you may want to add even more padding, but it is highly recommend you define the padding yourself, otherwise you'll be leaving it up to different browsers to decide for themselves. For even more consistency across browsers, you should also define the border yourself.