Image --> Image Size. I strongly suggest you watch some basic tutorials....
Mylenium’s advice to check out some basic tutorials is right on the money. Regarding your immediate problem, it sounds as though you are asking how to resize an image that appears as a layer over anther image. If that is the case, choose Edit > Transform > Scale. You’ll see a bounding box. If you cannot see all four corners, choose Cmd+0 (that’s zero, not oh). Hold down the Shift key to maintain proportion and drag a bounding box corner mark diagonally to resize the layered image. Last, hit the Commit check mark in the upper right corner of the Options bar.
Ctrl+Alt+I is the shortkey