what do you want to resize?
you can get rid of the main window, equaliser or playlist if you want - there's buttons for that, or right click on winamp and select in the menu.
you can lengthen or shorten the playlist window by clicking in the bottom right corner.
you can also make a slimline player by double clicking the playlist window.
if you want to make it bigger, there is a double size option, in the right click menu and also in the main window as one of the little letters next to the graphical display.
you can't actually stretch the windows around though.
EDIT (can stretch, but only the playlist window)