May program optimisation and analyses, such as array-bound checking, termination analysis, etc, depend on knowing the size of a function's input and output. However, size information can be difficult to compute. Firstly, accurate size computation requires detecting {\it size relation} between different inputs of a function. Secondly, different size information, and thus slightly different computation. Literature in size computation has mainly concentrated on size checking, instead of inferencing. In this paper, we provide a generic framework on which different size variants can be expressed and computed. We also describe an effective algorithm for {\it inferring}, instead of checking, size information. Size information are expressed in terms of Presburger formulae, and our algorithm utilises the Omega Calculator to compute as exact a size information as possible, within the linear arithmetic capability.