For a pistol (not revolver) as mentioned the S&W 61 Escort is the smallest, but my choice is the Beretta 21 Bobcat which is among the smallest in quality choices. I had a couple of Jennings in the early-mid 60's... never again.
The Escort was discontinued in 1973 after only about 3 years, so may be harder to find mint, I've seen them run from about $300 up $700 to buy depending on condition, etc. While a little bit smaller overall I believe it is thicker than a Bobcat in the slide area at least, is DA only with a 5-shot mag. I don't know whether it can handle the hyper velocity ammo, and there are tales of reliability issues that may have had some to do with its demise after going through 4 versions in only 3 years.
My Beretta Bobcat is my second. Not much bigger in profile than an Escort, drops in a pocket with ease, with a lightweight holster not noticeable as a pistol at all, almost not even without the holster in the shorts I wear. Both DA & SA... DA even from half cock with the safety off. With it's 7-shot mags and tip up barrel you can easily start with 8 (so a second mag gives you 15 - 33% more than the Escort w/2), and it will handle the high/hyper velocity ammo. This is my second I just got to replace the one I should have never sold. I never had any reliability issues even with cheap bulk ammo with the first even though they say high velocity minimun in them for reliability. In mine now all mags are loaded with CCI Velocitor's that are rated in the top choices for personal defense. The one I just bought is the higher grade discontinued polished blue model from the early 90's, is in mint condition metal with a few small bumps in the beautiful wood grips and I got it for $230. You can find brand new or near mint matte 21's for about $300 - stainless for about $60 more if you shop around, maybe less if you luck into one like I did.
The Walther TPH is quality, but is well known for it's bite (slide), you'd be looking around $500 plus for one and over $100 per for extra mags.
The others I would call quality are mostly larger - and still others I would call junk.
All just IMO - YMMV