In the Photo Library activity, you can use the Zoom toolbar to change the display size of the Photo images displayed in the client area of the Photo view.

Photo Zoom Toolbar
Click on the Increase and Decrease size buttons (1) to move through the standard display sizes or select the size directly from the drop down list of sizes (2).
The Fit to Window size will display the Photo image within the dimensions of the client area of the photo view.
Selecting the Custom Size displays the Custom Zoom Window, as shown below:-

Custom Zoom Window
1. Display Size - type a new display size in the display size field as a percentage of the actual size of the Photo image file.
2. Zoom size marker - alternatively drag the Zoom size marker left or right to increase or decrease the display size of the Photo image file.
3. Zoom increment - type a new value in the Zoom increment field to set the incremental value used to increase or reduce the display size.
Select the Exit button to close the window, or select a standard size from the Photo Zoom bar to return to one of the standard sizes provided.