Yes this device is meant to be hidden out of view mainly outdoors. Size should be the limited to the size of the phone itself and any casing you may need for environmental concerns. This does not limit you, if for example you need to utilize a probe or solar panel. It will just need to be concealable. When thinking about the mission of employing five sensors/phones think what you would want to be carrying in a field environment and how often you would want to replace the sensor battery. Emplacement and servicing of a sensor is the most critical time for an operator on the ground as it needs to be put into place and do its job and not compromise the operator emplacing it, so time battery longevity comes into play. I just wanted to give you a perspective, I hope I answered your questions, please follow up if necessary, thanks!
Tom C Answered question