Changing Blog Featured Image Sizes Post-Theme Change

Hello - we are using the Photolio theme on http://www.e3ngage.com. Our old theme used to require featured images for blog posts to have 600x600px dimensions to display properly. This new theme has 542 x 250px as its featured image. Is there a proper way to resize the featured image dimensions so that the new theme now uses 600x600px? The idea of going through all those old blog posts and changing the image size is daunting. Please advise how best to proceed.

Best,

AD