If you are thinking a day pack, or a hike, I dont know that I would be going whole hog on the 1st aid kit. Basic tape, bandaids and such will cover you in most cases. Look up the BSA first aid kit that they are supposed to carry. You can add or take away stuff, but the kit covers it pretty well.
Mine got expanded simply because there were 12-16 boys in the troop and we were hours away from anywhere with no fast way to get there and a lot of places where there was and is no cell coverage.