sorry i could not find an exact number anywhere but by looking at several pictures I would estimate about 5-6 inches by 4-6 inchest.
I would also guess that the card itself would not be any "taller" (when installed on the board) than a video card.
If you are worred about it fitting in your case, I would myself would be willing to assume that if your case can fit an Extended ATX board, you could fit any graphics card and therefore you could assume the thundbolt card.
EDIT: found a couple of other pictures and I think the card could be 7 inches tall. I am only able to guess based comparing the MOBO specs to the size of the card however the angle of each picture is different so it is hard to be accurate.
Sorry