A magnetic resonance apparatus has a positioning unit and a patient bed. The positioning unit can store a number of positions of the patient bed and can automatically position in an imaging volume the associated examination regions established by a marking unit in a diagnostic data acquisition procedure predetermined by a measurement control unit. The positioning unit can detect dimensions of a patient by evaluation of the number of positions of the patient bed and transfer these dimensions to a monitoring module. The monitoring module adapts the RF transmitting power to the patient according to the dimensions.