So there are not any actual set in stone rules for an everyday carry handgun, everybody is different and has different situational needs. That being said I typically tend to stick to a set of requirements when it comes to selecting a firearm for an EDC. This has nothing to do with caliber or brand mind you, it’s a set of rules that can apply to several models but I find them to be fairly synonymous to a variety of pistols.
The first thing I look at is capacity, I want to carry a lot of bullets or at least as many as I can get away with before I start to drastically sacrifice other requirements. If I had to put a number on it I would say 15 is as low as I go in magazine capacity. I also typically carry a spare magazine because the last pistol fight I was in, I ran out and wasn’t carrying a spare; luckily it was enough to get the job done but it was a mistake I do not plan to repeat. Having more bullets means you can deal with multiple threats and stay in a fight longer (hopefully long enough to finish it). Generally speaking, in real life, people also miss a lot because of stress, adrenaline, movement, etc. Also, not everyone has the inclination to train to a professional (high-speed) standard and while I disagree with this line of thought I do understand it, so having more bullets is a good rule of thumb regardless.
Concealability is the next concern in my mind. A concealed carry handgun should be just that, well concealed, and certain pistols will cater to this better than others. Size, shape, and weight are defining factors here. If a gun is so heavy it flops around excessively or pulls at your belt line, it kind of sucks. If it’s so large it prints out the yin-yang then it’s obviously not concealed. Geometrically speaking, the shape can create uncomfortable “hotspots” for the carrier and cause odd printing. Some of this may not sound like a big deal but considering most people carry guns more than they shoot them, it is. Again this is dependent on the individual and I have found that even the largest handguns may be concealed under the proper conditions. That being said, a concealed handgun should be comfortable to carry and easily hidden without the worry of discovery.
Reliability is incredibly important to me, because I need it to work every time no matter what oddball situation I find myself in. Selecting a handgun with a proven track record is a good plan but as long as it is a functioning firearm that is inclusive to the individuals instincts then you’re golden. Part of this is training but a well designed firearm caters to repeatability and consistent performance. Carry what works best for you above all else because that is the definition of efficiency.