Micromotors can be used to build up complex microtools for internal medical applications as, for example, steerable catheters or optical and ultrasonic imaging system. The thinner and smaller the micromotors are, the less invasive is the implantation. However, miniaturization of motors implies some limitations in torque, speed, and efficiency. This paper theoretically analyzes the scale effects on torque, efficiency, and thermal behavior of high torque permanent magnet brushless DC (BLDC) motors with ferromagnetic core coils operating in different in-body environment. Using a finite element model of a two-phase BLDC motor, scalability laws are provided for diameters between 0.1 and 100 mm and current densities between 1 and 1000 A/mm2. Based on the impact of the cogging torque and overheating of the motor, scale-dependent operational limits are calculated. Operational threshold can be determined at the point where cogging torque becomes dominating over total torque, limiting the use of traditional iron-core motors in the microscale. Current density limits are provided based on three representative in-body thermal scenarios: respiratory tract, body fluid, and blood torrent. Maximum current densities and corresponding torque and efficiency have been obtained for different micromotor sizes considering safe in-body temperature operation as threshold. It is demonstrated that micromotors of sizes down to 0.1 mm diameter could be used in internal body environments with acceptable performance.