The size is 800 pixels on the shortest side. I submitted an image 1024x768.  That's within guidelines.  It says so here:

*> *Quote - Artwork Submission Specification(s):

Images must be JPEG/JPG format (or compressed into a .zip archive), 800 pixels on the shortest side, maximum 80MB file size. Animation formats accepted are SWF MOV MP3 MP4 WMV, minimum 320 pixels on shortest side, maximum 80MB file size (or they can be compressed into a .zip archive).*

I suggest that anyone submitting reads thePoser blog regarding image size

Smith Micro Blogger says: August 23, 2010 at 8:23 am

Postwork is allowed. Image sizes: 800 pixels is the minimum. As far as max size, you can upload a file up to 80 MB in a zip file. And it’s one image per person per theme. Happy posing!

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