Size (bytes) of a sampled signal
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.