Choose a System Unit Scale that is appropriate to your game and your personal preferences. (e.g. feet or meters work well for most "human scale" games)
NOTE: Make sure your Display Unit Scale matches your System Unit Scale - this will guarantee that the units exported to Blitz match the units you see in Max.
You should also decide on a Frame Rate to use for all animations. NTSC or PAL are good choices if you think you might ever render the animations to video. (Choose the video format appropriate to your region)