How can the size of an input text box be defined in HTML?

Try this

<input type="text" style="font-size:18pt;height:420px;width:200px;">

Or else

 <input type="text" id="txtbox">

with the css:

 #txtbox
    {
     font-size:18pt;
     height:420px;
     width:200px;
    }

The size attribute works, as well

<input size="25" type="text">

You could set its width:

<input type="text" id="text" name="text_name" style="width: 300px;" />

or even better define a class:

<input type="text" id="text" name="text_name" class="mytext" />

and in a separate CSS file apply the necessary styling:

.mytext {
    width: 300px;
}

If you want to limit the number of characters that the user can type into this textbox you could use the maxlength attribute:

<input type="text" id="text" name="text_name" class="mytext" maxlength="25" />

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