Yes! Simplified's instant image resizer was built for this exact purpose. Resize everything from Facebook designs and LinkedIn profile images, to banners for Twitter and screenshots.
To resize your designs:
1- Log into your Simplified account with your email or Google/Facebook login.
2- Go to design section and open your designs by clicking Projects on the left sidebar.
3- Click on your design, and then click on the Resize Artboard icon located in the toolbar that appears above your artboard.
4- Click the template size you prefer from the menu that opens on the right side of your screen or click on the Custom Size button to choose a different size from the default ones.
5- Click the close tab and you're ready to keep designing!
Here's a quick video tutorial on how it works: