6x 2/3AA sized NiCd cells, in 2 sticks configuration.
Don't have anything to measure here, but you can do it yourself.
Here is a pic:
The cavity in the MP7 is molded very close to the dimensions, and does not let you insert anything box shaped with the dimensions above.
Worked on a bunch of battery solutions for the MP7, and so far the best one ever is theses LiPos. They are inexpensive and deliver more than the stock setup.