According to this thread the largest allowed file size for renderings, fractals and other computer-generated artwork is now 25 MP - anything bigger than this will be rejected in the future.

In this thread I have tried to explain why it doesn't make sense to me that e.g. uploads of 40 MP files are rejected while SS is selling upsampled files of 50 MP and more; but finally it's Shutterstock's decision what to accept, what to reject and what to sell to their clients.

I've never seen a supersize picture, interpolated to 400% from the original file, so I can't say anything about the quality of these upsampling results. But I sincerely doubt that it will reach the quality of a non-interpolated file - that's why I have always uploaded renderings at the highest resolution I could produce without spending to much time.

I couldn't find any place on Shutterstock where a client is tought that "supersize" means "interpolated" and only "large" means "non-interpolated".

So from now on it seems to be wiser to scale anything down to below 4500x3000 pixels because only then your uploads will be upsampled to supersize 9000x6000 pixels, giving you a bigger chance when a client is comparing file sizes and has the choice between e.g. 6000x4000 (non-interpolated) pixels and 9000x6000 (supersize-interpolated) pixels ... I don't feel very happy with that, but it is what Shutterstock wants so I will play their game.

Thankfully other agencies are happy with being able to offer the original 50+ MP hi-res files so I will go on wasting hours and hours with watching my computer raytracing pictures at insane sizes.