This tutorial will explain how to change the size of a whole image as well as the size of one layer.
To change the size of your entier graphic simply click the window with your graphic in it and go up to the menu's at the top and select Image then Image Size and change the values height in width.
To change the size of one layer click the layer and go up to the top of Photoshop just under the menus and click the bot beside Show Bounding Box. Now go back to your layer and by clicking and holding your right mouse button of the handles (squares surrounding image) and just drag. When your done it will ask you to apply the changes you have made and simply accept.