Derringers are pretty small, but not terribly practical. A DA38 (made in .357 by American Derringer) is 4.84" x 3.3", and weighs 14.5 ounces empty. Effective barrel length, in terms of distance to the tip of the bullet, is 1.71". Add on the length of the bullet to get the actual distance the bullet accelerates.
In comparison, an S&W 340 is 6.3" x something, and 13.3 ounces. Effective barrel length 1.88" + bullet.
Kahr MK40 is a little under 5.6" x 4.2" (seems like Kahr pulls a Glock and only gives the slide length, not actual overall length) and about 25 ounces empty. Effective barrel length 1.88" + bullet again.
So a derringer is substantially smaller, dimensionally, than anything else in the same power range, but not any lighter. And with a minimal grip, hard to control. Triggers are heavy, too, in the ballpark of 18". And because of the impossibility of regulating such a small double barreled gun, the barrels print about 8" apart at 10 yards, just in .38 SPL. A harder-kicking round like .357 mag would be considerably worse.