Hi
- Yes the empty frame weight is in the region of 800g.
- The arms of the Q800 are longer (800mm motor to motor distance) than the Q650 but due to the closer spacing of the motors in the Hexacopter configuration the max props you can use are about 14 inch, you could possible use 15 inch but as I dont have an assembled frame here to confirm the tips of the props could touch. But most 800mm size hexacopters max prop size is 15 inch.
- I would say that 3Kg would be fine but I dont suggest going much over this weight.
Also @Louise has shared some great pics and tips on her Q800 build so might be worth having a read before building the frame 
If you have any other questions about the frame please let me know!