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:
Log into your Simplified account with your email or Google login.
Open your designs by clicking Projects on the left sidebar.
Click the Resize Artboard icon on the toolbar that appears above your artboard.
Click the template size you prefer from the menu that opens on the right side of your screen.
"X" out of the editor and you're ready to keep designing!
Here's a quick video tutorial on how it works.