In this example of code; the textbox name is "username" and the size =20.
To find the code in your html: If you are using frontpage you could just double click the text box and change the size in the box that pops up.
If you are using some other editor then just click the text box once and then click on the html view and change the size value to a smaller number.
When you click on the textbox in normal or design view the html for the text box should be highlighted for you which will make it easier to find.
<form method="POST" action="Login--">
<p><input type="text" name="username" size="20"><input </form>
Hope this helps. If not let me know what software you are using. -Jason