  Estimation of software reliability often poses a considerable challenge,
particularly for critical softwares. Several methods of estimation of
reliability of software are already available in the literature. But, so far
almost nobody used the concept of size of a bug for estimating software
reliability. In this article we make used of the bug size or the eventual bug
size which helps us to determine reliability of software more precisely. The
size-biased model developed here can also be used for similar fields like
hydrocarbon exploration. The model has been validated through simulation and
subsequently used for a critical space application software testing data. The
estimated results match the actual observations to a large extent.
