The Key Defender has a profile about like a 3/4-sized mini-mag lite in length. In fact, the little removable mini-mag lite pocket clips fit it, so it can be clipped to the pocket. It has a flip-away safety so no concern of accidental discharge when carried in the pocket like a flashlight. My wife carries one in her pocket or purse. It's small enough there isn't a good excuse not to carry it. The downside is minimal range (~10 feet). There are also training cartridges for it, to aid in learning to draw and deploy the device. In my opinion, since the Spitfire went away, the ASP Defender series is the best choice to meet your criteria.
Of course, I do go bigger myself - what I carry right now is this:
https://www.copsplus.com/sabre-sabre...delivery-spray - The H2O version is easier to decon if you accidentally hit yourself with it. Still effective as a 10% OC with 1.33% MC and it has a UV marking dye in it.
That said, this is a big canister of spray compared to an ASP Defender sized package. I usually just stow it in the pocket of my "shoot me" concealment vest. My three biggest threats I carry OC for are aggressive panhandlers, including people who may approach my vehicle and refuse to back away, aggressive dogs (especially at the dog park), and aggressive dog owners (especially at the dog park).
The latter two are actually my biggest concerns. I've not seen people act so irrationally about someone grabbing an aggressive animal as they do when you grab their "wonderful pooch". Poochy wouldn't hurt a fly as he grabs a hold of a smaller dog and tries to do his best imitation of a petulant child breaking a toy...And if you move Poochy out of the way, Poochy's owner, usually mom, seems to think she is entitled to harass and/or assault you for making sure neither your dog nor Poochy ended up getting a lethal injection.