A good way to keep the size of a FAK manageable is to limit the FAK itself to a few critical items that cannot be readily itemized - large sterile dressings, elastic ACE bandages, and meds, for instance. Think through those items you will more likely improvise or substitute - I have tweezers on my multitool, and always at least one flashlight in my backpack. On a long hike or backpack, you simply can't afford needless redundancy. Fortunately, knowledge and skills are extremely lightweight.