A77 - Film Back measurements
Put your A77 on a tripod, disable steadyshot for both photo and video, take a photo, then take a short video clip without moving the camera. Import the photo and a still from the video in Photoshop, superimpose them, and you'll see the crop. The active area in photo mode is known from the manual, so you should be able to easily calculate the area in video mode by using the ratio.
And remember to shoot video with steadyshot off, as the active area will otherwise be shifted around depending on movements, and your software might not appreciate that at all.