When choosing a first aid kit you must take into consideration several factors. Where it will be used? Do you or someone in your family have special needs, and how it will be stored?
Most accidents occur in the home with children. This makes having a fully stocked kit available just common sense. The best may be one that includes instructions on what to do with the materials you have. Some are so sophisticated that there are audio instructions. Even if you have called for emergency help the operator can walk with you through what to do until help arrives.
In everyday travel from home to work, or school you should have a kit. It can be something small in a purse. Every parent knows when taking kids to the park anything can happen from a small scrape to something more severe like a fracture. Usually all that is necessary is a kit that has bandages, of course, some antibacterial ointment, and cleansing wipes. This you can create yourself or purchase ready made for those accidents that didn't occur to you.
A camping or backpacking trip may require something more extensive, depending on where you are going and how far you are from available help. If you are going to a place with snakes make sure yours includes a venom extraction kit, it may save someone's life. If you will be near your vehicle you can just keep it in the trunk and it won't matter the size or heaviness, but if you are hiking something more lightweight and waterproof may be necessary.
Each family has its own special needs. Perhaps someone in you family has a minor heart condition be sure to include necessary prescriptions or even an AED (automated external defibrillator). Your kit may never be needed to save a life but it can keep a person hanging on until help arrives. Remember all members and their individual conditions.
If you have a kit with instructions for different types of emergencies you will be able to help as long as you remain calm and do not panic. Read these ahead of time to familiarize yourself with what will be expected of you. To really be sure you are ready, consider a basic class for first aid and update your knowledge each year.
In order to select a first aid kit, review the different sizes and what is included. Judge what things you may need to include on top what is in there already. Check expiration dates on all your supplies and be sure to replace what is used so as never to be caught without the tools necessary help those you love in case of an emergency.