Big Huge Labs has some useful Flickr tools, one of them is the ability to show a Flickr photo large on black background. This is a fine way to present your Flickr visitors a quick link to a large detailed version.
If you want your visitors to view a large version on black, you have to provide a large enough version :) But how large should it be, or to phrase the sentence better: how small can it be, and still be viewed large on black?
Here comes the weird part: I have seen images as small as 1024*768 px that show up fine in "View large on black", but others in 1280x1024 px that don't...?
I have no idea why this is so, but to be on the safe side, you have to upload an image that is big enough to get a native "large" version and a bigger "original" version.
The sweet spot for Flickr to create that large version is 1281 px on the largest side, this is the versions that will be created if the image is 1280 px on the largest side.
So if you want to be sure that your "View large on black" link on your Flickr photos work as intended, the longest side has to be at least 1281 px :)