AirTags are 32mm x 32mm x 6mm. As for the inner-workings of AirTags, it'll use the Find My network from devices to send the UUID of the AirTag to the Apple servers for pinging. No other data from the tags get transferred. My guess is there's just a BLE beacon broadcasting that at any given moment that the iPhones pick up on.
Tags can also pick up information on other nearby tags, so it can kinda hop off one another. So when tags come in contact with other tags, they'll pass that data along.
Apple is expected to unveil its AirTag tracker some time this year at a price of around $39. There are rumors of a possible event in April; however, it's unclear if the accessory will make its debut at the event. You can download the iClarified app or follow iClarified on Twitter, Facebook, YouTube, and RSS for updates.