To make a document the same size as another open document, while in the Image Size or Canvas Size dialog box, select the other open document from the bottom of the Window menu (PS will automatically fill in the values).
One can always select Image > Canvas Size in order to numerically add or subtract to the width or height of your 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: If you can't drag the crop outside of the image because the image windows is in the way, try zooming out or using full screen mode.
To add transparency around the image (instead of filling the added space with the background color) convert the Background into a layer by dragging the Lock icon to the trash before cropping.