Hero images should be 1400 x 500 pixels.
Titan automatically creates various images sizes. Image copies are created and re-sized based on the longest edge of your photograph, maintaining the aspect ratio (with the exception of the thumbnail which is cropped square. Titan's image sizes are configurable. The default sizes are below.
Large - width 640px
Medium - width 320px
Small - width 160px
Thumbnail - width 80px height 80px