I'm using 6.3.5000 in IE 11 .0.9600.18618.
For some reason page does not open to the full 100% height anymore. I test this on different machines completely in and out of the office and all suffer from the same issue.
I did some googling on this and there may be an related issue with radWindows being opened in a table?
The 'edit a page' property (when you click edit page properties and title) is a Sitefinity page in the back-end so I don't have any chance to influence the CSS. Again, this is on all machines running IE11. Was possibly because of an update?
When digging through the CSS rules, if I change 100% !important to (for example) 750px !important the window stretches properly. My guess this might be related to rad tools being used in tables (seen similar issues discussed with radtools and tables)