Hello Malgo - I completely agree with your observation wrt the beta site. I upload a bigger shot specifically for viewing on that version but make sure that it fits the page. I get really frustrated when I see massive shots posted as I have to scroll up and down or back and forth or look at the shot on the newer version of TE. I have participated in a few threads wrt the 'newer' upload options and remain of the opinion that clearer instructions should be given and that the upload size for the beta version should be limited to the 1200-1400px size you mention. In an ideal world it would be great to have the whole TE site revamped to bring it more in line with the more sophisticated and attractive designs of other sites. This though doesn't appear to be an idea that the site owners have any appetite for.
I have got to the stage where I don't bother with scrolling through a massive image unless it is posted by someone whose work I follow or whose TN looks exceptional. Then I usually switch to the newer version of TE... not very user friendly but there is little option.