Hi. I am a retired fire lieutenant-medic and LPN, and also a first aid and CPR instructor.
The size of the first aid kit you need depends on the number of people you normally have assembling together at one time. Professionally-created first aid kits are designed for specific numbers of people. If you are a small, family-oriented type of church, a small, simple first aid kit is all you would need. I can recommend a certain one from a certain store, but I'm not sure if that's allowed. However, I will tell you that you can buy this kit from the nations's #1 department store, in their health and beauty section, usually for under $10. The kit's maker has the initials J & J. I keep one in our house, though I still carry a trauma bag with me. The price you would pay for all of the parts, pieces, bandanges, etc inside, if bought separately, would cost at least twice as much. It is a very good buy.
Or, I also recommend buying a good tool box, and buying the items you would need to fill it, as buying a ready-made kit will cost a lot more than you might spend in buying the items yourself. I recently bought a professionally-created kit, meant for no more than 50 people, and it cost me $50. And you don't get as much as you might buy for the same amount if bought in a discount store. Look around for the best bargains on bandages, gauze pads, roller gauze, instant cold packs, etc, etc. Let me know what you think. I can get you a list of what you should have in your kit. Let me know the umber of people who congregate together at one time.
If you have quite a number of people, another great, but expensive item is an AED, (automatic external defibrillator) for dealing with heart attacks, cardiac arrests. All places of assembly should have one.