The length of a graded module is the difference between the largest and smallest indices of occupied spots. Chain complexes are graded modules, so this function applies to them, too.
Examples
i1 : R = QQ[x..z];
i2 : C = res coker vars R
1 3 3 1
o2 = R <-- R <-- R <-- R <-- 0
0 1 2 3 4
o2 : ChainComplex