Having tried it, I've just remembered why I needed to use the setSize function...
The invisible buttons will be appearing over text fields, but since the amount of text in the field differs on each one, I wanted to be able to get the width and height of the field and set the button to the same (so the button isn't twice the size of the text field).
The reason I can't just place the buttons over the text fields manually is because its all created dynamically in a FOR LOOP.
Also, since text fields won't allow an onRelease or onRollover function to be added (or at least it didnt work for me...), I tried using text areas which allow it. However I believe I had the same resizing problem because I was basing the resizing on the length of the text but because its not a font like courier new, the size of character is different so saying 'lenght * 3' or whatever won't work because 'A' takes up more space that 'i' etc etc...if you dont get what I mean, its not important...I'm just babbling!
This is a strange one because all I want to do it resize what is essentially a movie clip...even when entering values manually such as setSize(100,100) somewhere into the loop, it still doesn't work which is making me think the problem lies elsewhere...