1- It's the width of the screen/window, but only vertically high enough to note that an 'X' (ostensibly to be clicked to dismiss it) is at the right-hand end. Resizing the window doesn't reveal any more of it.
2- It's appeared on any regular page here (at least today)
3- Desktop: CompaQ CQ 2011 (new 2011)
4- MS-Windows 7 Home Premium (custom to HP/CompaQ) 64-bit with Service Pack 1 (they never did any more, and Win7 is condemned to the wilderness as of Jan 2020)
5- Firefox (just updated to v.64)
Since I haven't looked here in over a year, I have no idea why it's there. The required thing about using cookies (to satisfy that EU rule?) usually shows up at the bottom of a screen/window.