One can always select Image > Canvas Size to numerically add or subtract to the width or height of one’s image, but if you would rather eyeball it, try using the Crop tool. Drag out a crop marquee and release the mouse. Then, grab one of the anchor points and drag it beyond the visible image area. When the crop is applied,the area outside of the image and within the crop marquee will be added to the image canvas.
Note: to add transparency around the image (instead of filling the added space with the background color), convert the Background into a layer by clicking the Lock icon (on the Layers panel) before cropping.