From what I've seen, the majority of oneboxes are for inane things such as image macros, question dumping, and twitter quotes. Now, don't get me wrong, I enjoy this stuff quite often, but not when it's taking up a quarter or more of my screen!
Of course, oneboxing is also done for more useful things, such as links to questions, screenshots, and posting diagrams, but the problem this time is that the oneboxing can be too small! Stuff like screenshots are never readable when oneboxed, yet still take up 10 chat lines as an enormous thumbnail. For SO questions, the amount of text in the onebox is never helpful as it's only the first few sentences and often contains unreadable code.
Image taking up 1/2 of my screen (I don't mean to rag on Etienne, it was just the first image I found)
My suggestion to fix this is to keep all oneboxes small by default, and have a button to expand and shrink them to a larger size. Clicking on the image itself would open the link rather than expand it.
- Images would be a thumbnail (literally the size of a thumb nail).
- Animated images would not be animated in the thumbnail and would say "(Click to Expand Animated Image)" as a warning.
- Links to questions and articles would only show the title.
- Twitter posts would be formatted like normal chat quotes which fit on a single line.
- Youtube links would display the video title and not expand (seriously, the title is a better description than only a static image which is what we have now)
Here is how it would look for the first example image I posted: