Mouse guns are all about concealability, and it used to be that the smallest guns had to be in small calibers, typically .32 on down.
In recent times, we've seen some really good guns introduced that are easily small enough to be characterized as a mouse gun, but finally done in decent calibers.
The most ubiquitous is the Ruger LCP, and it's no accident that it's size and caliber (.380) coupled with it's cost of just over $300, it has sold somewhere around a gazillion or so.
The very most notable is the Rohrbaugh R9s which takes the mouse gun to new territory being that it's the same size of the LCP but in 9mm. Costing 3-1/2 times more than the LCP, it may never sell in such numbers as the Ruger, but it a very worthy pistol for those who can afford it, and worth every penny.
I see no reason any more to carry anything in .32 caliber or below.