This one too, slightly less cost:
https://www.amazon.com/First-Alert-C...dp/B003NE69CS/
...but probably the largest of the three options. You have to weigh (pun intended) the bulk and space taken up in your carry-on or checked bag, and also I'd think TSA would take more kindly to the small keychain variety than the full-size smoke detector form factor. At the least, they may question as to why you're schlepping around a "smoke detector." They likely wouldn't question a keychain dongle thingy.