If you just read data from data register of BMI270, the following is the bytes information:
accel data use 6 bytes, gyro data also use 6 bytes.
AUX sensor data can occupy up to 8 bytes.
Sensor time 3 bytes.
So if you need both of them, then in total 23 bytes.
If you read sensor data from FIFO of BMI270, then you can check the FIFO length register (0x24 - 25) to know how much bytes storeds in FIFO already.