Throughout Softimage, lengths are measured in Softimage units. How big is a Softimage unit? It is an arbitrary, relative value that can be anything you want: a foot, 10 cm, or anything else.
However, it is generally recommended that you avoid making your objects too big, too small, or too far from the scene origin. This is because rounding errors can accumulate in mathematical calculations, resulting in imprecisions or even jittering in object positions. As a general rule of thumb, an entire character should not fit within 1 or 2 units, nor exceed 1000 units.
The Softimage units used for objects also matters for creating dynamic simulations where objects have mass or density and are affected by forces such as gravity. For more information, see Size Does Matter [Simulation Basics].
Autodesk Softimage v.7.5