Serial Peripheral Interface (SPI) vs Two-Wire Interface (TWI, I²C) wire length
The limiting factor is the capacitance of the interconnecting wires. A pair of wires has a capacitance between them, measured in \$Fm^{-1}\$. Therefore, as your wire length goes up, the capacitance goes up. This means that, to a first approximation, you can think of the wire and the driver as an RC filter, so your nice square pulses no longer look like squares. You might be able to calculate the wire length, but there are a lot of variables, so a definite answer is unlikely. Some suggestions:


Think about using other protocols rather than SPI or I2C, which are designed for short distance PCB connections.
Use as low a frequency as possible for your application.
Use as high a drive as possible. There may be a "strength" register setting for the SPI driver, or for I2C use as low as possible value for the pull-up resistors.
Do some local processing where the IMU is, so you don't need the full data bandwidth back to the central controller.


Overall, a good engineering problem to be solved - lots of contradictory requirements, so spend a bit of time actually specifying your system, and then identify where you have problems. For example, do you need to read out at 8 MHz, or is 100 kHz enough?