Sorry if this has been addressed before, but I just discovered Export As (I've been using Save for Web and Devices).

The idea of exporting in multiple sizes simultaneously is quite appealing. I want to produce images for MM (800 pixels wide), for phone/tablet (fit into 2048 x 1536), and full size. As far as I can tell, Export As only supports relative sizes, and I frequently crop my images to oddball sizes, like 5792 x 7136 pixels. If I didn't crop, I could work out sizes that would do, but calculating them on the fly to get the output I want seems like more work than using Export As three times.

Am I missing something? Is there a way to say "Export As 800 pixels wide" instead of "Export As 0.33x"?

I don't know if you have any experience using actions (An incredible powerful, yet often under utilized tool that can save you a ton of time with repetitive tasks).

You can actually create an action to automate the resizing of your images so that each image is saved at various sizes. For example you can create an action that uses "Fit Image" to restrict size on long end (You will find fit image in Photoshop under the dropdown menu: File > Automate > Fit Image...), then also include "save as" commands for the file format in your action to complete the task. You can google how to use fit image, but basically you can use it to constrain images to a specific height or width.

You could also give Image Processor Pro a try. In Photoshop, it's File > Automate > Image Processor Pro and select from the various options there including the option of running actions inside it.