You mean your cell contents (HTML) are too large for the "Contents" preview area? That's kind of the point .. the preview area is of a fixed size to fit into the (already pretty cluttered) dialog. It's supposed to just give you a preview of what the contents are - and you click to view the full contents in another window.
The options would be to make the preview area bigger or add scrollbars. Making it bigger is really only a partial solution, since it will make previewing the contents better for some contents, but there will still be some that don't fit. Adding scrollbars would make it entirely viewable, but a little annoying and add more clutter. Plus, if you've got to scroll, you might as well just click it to see the full contents.
I'm reluctant to change any of this right now, but I want to make it as usable as possible. The size could be larger, but probably not too much. The dialog is pretty big as it is, and is approaching the max size (if I consider 1024x768 the minimum screen size). We've got about 100 to 150 pixels to work with vertically.