If want to place the Before and After pictures in a landscape-like fashion left and right side by side in Photoshop, then you need to double the width of the canvas.
You can select the measurement unit from the dropdown. In the example above, my Before image measures 600 pixels in width. To double the canvas size, I’ll need to enter a value of 1200 pixels.
In the Anchor section of the Canvas Size dialog box, you can move the point to where you want the Before image to be on your canvas.
I want my Before image to be on the left-hand side, so I’m positioning the point in the center left field.
Once you’re done, hit OK!
If you like to put the two photos side by side at top and bottom in a portrait-like manner, you would obviously need to double the canvas height.