- Crop your image first to the correct proportions. For example, if you want to enlarge to a 16x20, use the crop tool to crop to 16x20, but DON'T enter anything into the Resolution box.
- On your Photoshop menu, go to Image > Image Size.
- Make sure "Resample Image" and "Constrain Proportions" are both checked.
- Enter your desired height or width (the other dimension will automatically change to match).
- Enter your desired resolution.
- Make a note of the resulting pixel dimensions at the top of the window. It will be the larger size to the left of the numbers in parentheses. This is you target image size.
- Close the dialog box WITHOUT clicking 'OK'.
- Now, set your info bar to show document size. This will help you to keep track of the file size as it grows.
- Play the desired "res up" Action until you have gone one step past your target size.
- Undo your last step back one size.
- Open the Image > Image Size dialog again. Make sure "Resample Image" and "Constrain Proportions" are both checked.
- Type in the needed width or height and resolution again, but this time click 'OK'.
- All done!
If you know you are going to enlarge quit a bit, use the "Res up" 5 or 10 steps tool. It will bump up 110% five or ten times.