If you are seeing your table as taller than the viewport, then you are likely using IE. If you look at your page in Firefox, you will notice this is not the case.
In IE, the middle table cell is inheriting the height from the table, which is in turn inheriting it's height from the body. I doubt this is the correct behavior. I'm not certain that Firefox's behavior is correct either. Regardless, there is no "height" attribute for the TD tags in XHTML.
In XHTML you're not really supposed to be using tables for layout, but rather DIV's. There are many tutorials on the web for various layouts with DIV's and CSS. It's not easy. Differences between browsers make a table-less layout challenging. I usually have to use quite a few "workarounds" and CSS hacks.