i thought it was 284x244
Why? The forum guidelines say max of 800 pixels on the longest side so any 100% crop below 800x799 would be 'acceptable'. I agree that you can generally make a judgement of IQ on a smallish 100% crop of the kind of size you quote. Several small crops from different parts of the image is more useful than just a large central crop. The crucial factor is not the size of the cropped image, it is that they are 100% crops i.e. no resizing.
That is assuming you can ever make such a a judgement using a monitor screen - the ultimate test I suppose is a print at 300 pixels per inch or so, isn't it? Am I right there?
So much to learn!