Code:
<form>

<select name='size'>   
   <option value='size1'>size1</option>
   <option value='size2'>size2</option>
   <option value='size3'>size3</option>
   <option value='size4'>size4</option>
</select><br>

<textArea name='textArea' style='width:500;height:150'>
target text
</textArea>

</form>
I have a selection and textArea like the above.

I like to make it like the following.

(1) A user puts the cursur in fron of target text with his or her mouse.
(2) He or she drags the cursur to the end of target text.
(3) He or she selects size3 using the selection box.

Then,

"target text" becomes "<font size='3'>target text</font3>."

I hope I make it with your help.