Smooth button transition

hey there =]

iv been trying for awhile now to get a few of the buttons im using on a site to change dimensions when i hover

basically i would like the button background to change dimensions and extend length wise to the other end of the web page whenever the mouseover appears the lenght of the botton extends smoothly and also goes back to its original size smoothly without snapping back

iv read lots of tutorials (got adobe flash cs4 book) but its all about moving images and so forth not really dimensions

if anyone can point out a tutorial or knows how to resolve this