Is this battery operated? And will it fit in a standard mailbox? Is it humidity and/or water resistant? I want a smart sensor to know when the mail is delivered.

Yes it is battery operated and it will fit in a mailbox. I am not sure about the humidity but it is probably not water resistant.I think you should consider a door sensor that is a simple open or close switch. When the door opens your system would know the same for when it closes. I know with my Smartthings I cold get it to send me a text when the mail box opens. The door sensor should also be cheaper. I have several door sensors that have been in place for well over a year that still show 100% battery level a motion detector that I put in at the same time is at 70%. You can do the same thing with the motion detector but I would consider the door sensor that is assuming that the mail person closes the door all the way.