Open your layers palette, press and hold control and then click on each layer. This will select them all.
Then press control and T. This will select the transform tool, with this you're able click on one of the boxes in the corner and change the size of every layer.
If you want to make it bigger go to edit > canvas size and increase the size. Then you can enlarge your image using the transform tool. If you enlarged your canvas too much use the crop tool to get rid of the parts you don't want.
Let me know if you have any problems.