The width of these items is defined by the size property for the text field inputs and the cols property of the textarea inputs.
For example, find the line of code that looks like this:
<textarea id="ProductLongDesc" cols="32" name="ProductLongDesc"/>
That's the definition of the textarea. If you change the cols="32" to something like cols="28" the width will change on that.
For the text field, take a look at this line:
<input id="ProductSKU" type="text" size="32" value="" name="ProductSKU"/>
That's the definition of a text field. If you change the size="32" to something like size="28" the width will change on that.