1. In a computer system comprising a plurality of instructions having multiple instruction operation sizes, and a source program to be compiled, an automated method implemented on said computer system for optimizing the instruction operation sizes to be used for said instructions, when generating said instructions for said source program while compiling said source program on said computer system, said automated method comprising the steps of: a) assigning a plurality of initial instruction ordinals representative of said instruction operation sizes to said instructions, upon generating said instructions for said source program being compiled, said assignment being made based on a set of predetermined initial instruction ordinal assignment policies, said predetermined assignment policies embodying logic for assigning initial instruction ordinals that are representative of minimal instruction operation sizes of said generated instructions, without regard to said assigned instruction ordinals' compatibility with one another; b) iteratively adjusting said instruction ordinal assignments until said assigned instruction ordinals converge into a set of assigned instruction ordinals compatible with one another, said iterative assignment adjustment being made based on a set of predetermined instruction ordinal assignment adjust policies, said predetermined assignment adjustment policies embodying logic for avoiding unnecessary assignment adjustments to instruction ordinals representative of the larger instruction operation sizes; and c) mapping said compatible instruction ordinals assigned to represented instruction operation sizes, conditionally inserting additional instructions to the generated instructions if necessary, based on a set of predetermined mapping policies, said predetermined mapping policies embodying logic for identifying the instruction operation sizes represented by the instruction ordinals and the necessary instruction insertions.