I need to double the sizes of many images for retina devices. It takes too long to do it by using a calculator to do the math and manipulating the values in the width/height panel at the top. Is there a one key shortcut to speed this up?
You could transform one image, by scaling it up to double size and then use the 'transform again' shortcut to do it on other images in the same doc - 'ctrl + D' or 'cmd + D'.
As far as getting the calculator out, you can use mathematical operators in the height/width panel in the top. E.g. 150px * 2
If you are using the Save for Web option, you can let the Save window do this for you. You can keep your image as is, but save it twice its size.
Just enter 200 in the percent field. Make sure you tab out or exit the field, so the window actually gets the value and re-sizes your image. Then, after you see in the preview that it has doubled the image dimensions, click on Save.