http://www.passportsr1.com/sr7.htm
Here is the website for the detector. You can see there is two seperate pieces to mount. A front radar and a front lasar unit.
The two pieces measure as follows:
* Radar Receiver Unit: 3.75 x 3.15 x 1.75
* Front Laser Unit: 4.65 x 1.8 x .80
And here is a pic of the front radar sensor... So I think it is far to big to put behind the rear view mirror.