Considerable efforts have been made to study heat exchanger microreactors. However, the starting point in :design for the selection,of an adequate length scale that takes into account both the maximum temperature rise and pressure drop, important criteria for the holistic design of such equipment is still missing. An attempt has been made in the present communication to provide such relationship over a wide range of operating variables. Fast and simple guidelines are discussed, for beginning to design a heat exchanger microreactor for exothermic reactions where the maximum temperature deviation inside the reactor, called the hotspot temperature, can be predicted by calculating the overall heat and the maximum heat, release rate. The applicability of the proposed guidelines was tested using experimental and simulation data from other authors.