Once an image is imported or opened in Photoshop, it can be resized by selecting Image/Image Size. Also, the space around an image can be increased or decreased by selecting Image/Canvas Size. Below is a description of these interfaces:

Use the ALT key (Mac- OPT key) to change to demagnification and reduce the image to 50%.

Note: For demagnification, you have to keep the ALT/OPT key pressed while clicking the mouse.

3. From the menu bar, Select Image > Image Size. In the Image Size Window, change the value of the the Width Column in the Pixel Dimensions Box to 350 pixels. Choose Ok.

Note: You may have to click the Resample checkbox to access the Pixel Dimensions Box.

Next, open Image > Canvas Size and move the Anchor to the upper left i.e.click on the upper-left most cube. Change the canvas size to 450 pixels (make sure pixels is the unit of measure indicated)