Hi, I don´t know if I understand you correctly, but click (left) in the middle to get a brown frame. Then you can enlarge the image by draw the sides or the corners. While you do that, hold Ctrl+Shift to get the right proportions. Yoy can move the image (in the brown frame mode) by click (left) in the middle circle. Good luck!
There are several ways you can do this, but they all depend on selecting the image, not the frame (waht Jan is referring to as the brown frame mode, but if you use multiple layers, the color may not be brown).
One is to drag the size of the image directly, as suggested, another is to use the scale fields in the control panel to enter percentages or even absolute dimensions (if you include the unit of measure). There is also a pair of keyboard shorcuts, Crtl + Alt (windows) or Cmd + Opt (Mac) and the < and > keys which will scale in 2% increments withthe postioning based on whihc point is selected in the transform proxy.