This paper addresses the challenges of designing high speed synchronous reluctance machines (HS-SyRM). First, a comprehensive design methodology is presented trying to take into account the electromagnetic, the thermal and the structural design aspects. Then, four different stator current calculation criteria are introduced and compared designing HS-SyRMs for a wide range of maximum speed. Indeed, the effect of constraining the current density, the Joule losses, the stator losses or the total losses is deeply investigated analysing how the performances change with the selected independent design variables, namely split and magnetic ratios. The performance boundaries of the optimal geometries as the maximum speed increases are analysed when adopting the four current calculation criteria with particular emphasis to the thermal implications.