The recommended image sizes are listed below (in px):

Posts Thumbnail: 60 x 60 px

About Thumbnail: 600 x 400 px

Products Thumbnail: 235 x 235 px

Product Thumb Layout 2: 380 x 240 px

Slider : 1920 x 630 px

Testimonial : 317 x 347 px

Portfolio : 380 x 460 px

Team: 370 x 486 px

Image Full (without sidebar) : 1140 x 498 px

Image with sidebar : 750 x 400 px

Team Thumbnail: 487 x 527 px

Blog Thumbnail: 280 x 255 px

Events Thumbnail: 255 x 255 px

Events Page: 880 x 450 px