The size attribute defines the width of the <input> and the height of the <select> element. For the input, if the type attribute is text or password then it's the number of characters. This must be an integer value of 0 or higher. If no size is specified, or an invalid value is specified, the input has no size declared, and the form control will be the default width based on the user agent. If CSS targets the element with properties impacting the width, CSS takes precedence.
The size attribute has no impact on constraint validation.
By adding size on some input types, the width of the input can be controlled. Adding size on a select changes the height, defining how many options are visible in the closed state.
<label for="fruit">Enter a fruit</label>
<input type="text" size="15" id="fruit" />
<label for="vegetable">Enter a vegetable</label>
<input type="text" id="vegetable" />
<select name="fruits" size="5">
<option>banana</option>
<option>cherry</option>
<option>strawberry</option>
<option>durian</option>
<option>blueberry</option>
</select>
<select name="vegetables" size="5">
<option>carrot</option>
<option>cucumber</option>
<option>cauliflower</option>
<option>celery</option>
<option>collard greens</option>
</select>