Techniques and devices for extending a piston and/or compression unit, for example connected to a medical device such as a mechanical CPR device, to accommodate different sized patients, are described herein. In some cases, a piston of a mechanical CPR device may include an inner piston at least partially slidable into an external piston sleeve. In one aspect, some aspects, the piston includes sleeves which can move relative to each other to extend the piston. In additional aspects, the compression mechanism may also extend downward toward the patient. In all aspects, the change in length of the piston may be detected and used to modify movement of the piston, for example to more safely perform mechanical CPR.