If you want to change the size of a layer in your sig,
select the arrow on the left panel, then click "edit" then "free transform"
with that youll be able to move and change the size of the selected layer. (layer are displayed on lower right panel by default.
If you want to change the size of actual image (the complete pic)
then go in "image" and then "image size" and there you change it like you want.
I have Photoshop 8 cs but thats probably the same thing.
Hope this help