The first thing on the list should be proper training.  I encourage everyone who regularly participates in wilderness trips of any kind to take as a minimum a Red Cross wilderness first aid course and better yet the WMI or NOLS wilderness first responder course.  Without proper training you may not know how to use some of the items that will be suggested.
With only basic training the first aid kit described in the Boy Scout of America First Aid merit badge booklet would be suitable for most common injuries or emergencies.  Campmor sells several first aid kits.  The most expensive one is very comprehensive but probably heavier than what you want to carry.
I put together my own based on my own training/experience.  It packs into a 8"x6"x4" tupperware plastic storage tub with waterproof snap on lid.  I carry prescription antibiotics, benadryl, epipen, immodium, chewable peptobismol, suture material/forceps/scissors, local anesthesia, irrigation syringe, baby aspirin, latex-free gloves, rescue breathing barrier, surgeons hand scrub, sterile cling wrap, ace bandage, 4x4 gauze pads, roll gauze, mole skin, big triangular bandage, assorted cloth backed bandaids, and the one item I almost always run out of on group trips: 600 mg Rx strength ibuprofen. I put a sheet with copies of my training certification and RX's for the meds in the box too.  The anesthesia and syringes are not strictly legal, but with the certification I have and an explanation of where I am going, I have never had a problem.  I just make sure everything else I have or am doing is legal or I am sure a customs agent or ranger would nail me for it.  I'm probably forgetting something, this is off the top of my head.  But again, the most important thing you can carry here is knowledge and training of how to assess and manage emergencies.