I recently had to downsize many web screens that I made on Illustrator, and resizing the already designed artboards with objects on them seemed like a pain.
Here is a very quick hack to resize one or many (already exported) image files (at least on mac). You need to use terminal, but it is very easy:
Make and export your images in Illustrator (or PS or whatever you are using) in the maximum possible size you would need. Create a new folder with copies of these images (or just make a copy of the image itself if you only need to resize one or two images). Open terminal, navigate to the folder of the image and run the command:
sips -Z xxxx <filename>
Replace "< filename >" with the name of your file, and "xxxx" with the maximum dimension you want the image to have. "-Z" maintains the aspect ratio of the image.
If you are new to terminal, < filename > can also be replaced with the full PATH of the file, if you can't be bothered to navigate to the folder. (Tip: drag and drop files and folders from the Finder into Terminal instead of typing the whole name out)
The above will resize one image. To resize multiple images, make sure you are in the right folder and use:
sips -Z *.<file extension>
This resizes ALL images in the folder you are in with the given file extension.
CAUTION: sips does not create a copy, it will directly resize the image, so make sure to copy your images first if you want to keep the originals. For quality, this works best ONLY to make images smaller, not bigger.
sips -Z 1024 *.png
This will resize all my images that have the extension .png in the given folder to a maximum dimension of 1024 px, maintaining the aspect ratio.
For more information type in terminal: