I have a fondness for "mouse guns" and own several. However, there seems to be some disagreement on just what a mouse gun is these days. My definition is a small, pocket sized pistol in no caliber larger than .32 acp. And even then the .32s would depend on overall size. IMHO the Beretta 3230 Tomcat is the biggest thing that can be considered a mouse gun. And even that is pushing the definition.
However, many modern writers now want to include .380s and even some 9mms in the mouse gun category. Just because its small for its caliber, does not make it a mouse gun. True mouse guns are generally in .22 or .25 caliber and will fit in the palm of your hand.
I, myself, prefer those in .25acp. Simply because the center fire .25 is much more reliable than the rimfire .22s. There are some really nice .22s out there, but choose your ammo carefully.
During the 1990s I was a Boy Scout Leader working at the District/Council level. This often had me out at night in some not too friendly parts of town. Its darn near impossible to hide a decent sized gun in a Scout uniform. On these nights my PSP-25 was always in my pocket. I never felt under gunned with it.
One must also change tactics with a mouse gun. Forget center mass. Odds are this gun will be used at bad breath distances anyway. Instead, stick the gun right in the BG's face and pull the trigger. Several times. While the bad guy is distracted by the blood and pain, you run like Hell! Always remember that the true purpose of a self defense gun is to save your life. Doesn't matter if the BG is incapacitated or not as long as you get away unharmed.