Size (bytes) of a sampled signal I am a beginner and maybe I have the false idea but I will thank your understanding, my background is not so on the DSP side. 

I have a KX134-1211 accelerometer that delivers a digital output. 
It is triaxial 16 bit sensor and delivers at 2 axis 8500 Hz as signal bandwidth.

This means the sensor fmax is 8500 Hz right? Since it is already digital how much sampling rate do I need to get the maximum signal banwidth, 8500 or 17000 Hz?

If I sameple the sensor for at 8500 during 1 second I will get 8500 values, how much bytes is this, I want to calculate how much size each recording will take.
Please have a look at Page 5 of the DataSheet here.

It says the signal Bandwidth is 8500Hz only when output data rate is 25600Hz and Low Pass Filter setting = 1. And that the Bandwidth changes based on Output Data Rate. 

So, I think you do not have to worry about sampling rate, since ODR(Output Data Rate) is 25600Hz and that means 16-bits(2Bytes) readings coming at this rate.

So, 1 second of recording will require : 1*25600*2Bytes = 51200/1024 KB = 50KB, of memory space. And, I am not sure but I think this is just for X-Axis measurements by the accelerometer.