I posted this in another thread but this one looks like it could use the info too.
I used digital calipers to take the measurements. They are: 65.00mm x 43.80mm x 4.30mm.
If someone else has an Eris and some good calipers, we could compare notes.
Note in the next pic the two notches in the left and right corners of the battery. They line up with two tabs on the battery slot. Any replacement battery will need these notches to work. Also notice the moisture sensor on the right of the battery.
Some more info on the battery back:
Model #: BTR6300B
3.7 VDC
1300mAh
4.81 Whr (watt/hour?) Li-ion polymer battery
Phone model: ADR6300VW
Is the Eris battery covered in a thin metal? The Inc. battery is.