Gee, where to start?
Tuxedos, sometimes called "black tie" or "semi-formal" by those of us of a certain age, are cut differently than are business suits. There isn't a lot of room in the pockets for a lot of crap. If you have much more than a money clip, credit card, and driver's license in one pocket, and a car key and house key in the other, you will have lumps in your trousers pockets (and not the kind that is going to get you a lot of extra dates, either, if you follow my drift). It just isn't going to look "right". Many tuxs do not have rear trousers pockets. (The trousers for formal wear, also called "white tie" or "tails", frequently do not even have front pockets. So if you are going full formal, somebody else is going to have to carry your stuff.)
So, if you absolutely must carry a flashlight when wearing black tie, you might check out
this item, reviewed by our friend at the LED Museum
here. Pay particular attention to the dimensions, and runtime of this little jewel. If you order it from the manufacturer, they will eat the shipping charge, so you come out a couple of bucks ahead of where you will be if you order it from the retailer also listed in the review.
If you really are looking for a really small flashlight, and want something with a bit more pizzazz than a $1 plastic / coin cell light from County Comm or wherever (not that there is anything wrong with those - they are great for what they are), this might work for you. FWIW. And, as always, YMMV.