There are three factors to consider in the choice of instrument size:

1. Each size instrument has a limit on the maximum weight of test part and fixture (there is no limit on maximum moment of inertia). An instrument should not be chosen with a maximum weight capability any larger than necessary, since larger instruments have poorer accuracy for small parts. Any size or shape object can be measured about any axis provided its weight does not exceed the rating of the instrument.

2. The smallest moment of inertia which can be measured with rated accuracy is primarily a function of the tare moment of inertia of the instrument. If the part inertia is 100 times the tare inertia of the instrument, then a small change in tare inertia will not appreciably affect instrument accuracy. If the part inertia is 1/100 of tare inertia, then a 2^oF change in ambient temperature will introduce a 0.5% error in the reading of part inertia due to the small increase in tare. This means that frequent re-calibration and tare measurements are necessary to get meaningful results if the test part moment of inertia is much smaller than 1/20 of the tare moment of inertia of the instrument.

3. The maximum CG height of the test object is limited by the stance of the instrument bearing. In the GB and smaller KSR series instruments, the bearing stance is equal to the bearing diameter. For the POI and larger KSR instruments, a second gas bearing is used to increase the stance so that greater CG heights can be accommodated.