Legio VI Constantiniani is a set of miniatures wargames rules aimed to allow large battles of Roman Late Imperial period (AD 250-600) to be resolved relatively quickly. It's meant for 6mm figures but could easily be adapted to larger (or smaller!) figures.
The rules are derived from the same designer's Comitatus set of Dark Ages rules, but slimmed down to allow larger battles to be fought in less time. Troops are classed a relatively small number of troop types with minimal differentiation based on equipment; the focus is on troop quality and, especially, generals' limited means of controlling their forces.
The rules are freely available from the designer's webpage, along with sister sets covering adjoining periods:
Legio VI Julia Augusta, for 100 BC to AD 250
Legio VI Macedonica, for 400-100 BC
Legio VI Diadochi, for Alexander and the Successors