There's a strange kind of logic at play that states the size of a first aid kit should depend on the number in the group. Admittedly, the more there are of you and the longer you are out there, the more likely you are to need a first aid kit... but the kind of wound that may need treating is largely independent of these variables. Putting it another way, the likelihood of needing the kit goes up with the number of people, but the kinds of accidents to anticipate are equally likely for all equally susceptible people. Much more important than scaling your medical kit to the group is scaling it to the accessibility of proper medical care and adapting it to the environment you are in. When trekking & travelling in the developing world, I carry a strerile needle set (despite my phobias!) - but when in the UK it is more important to carry a phone and get help fast.
In the end you have to make a cost-benefit decision - how much weight, space and funds are you prepared to give to cope with something that should never happen. And there's little benefit in carrying something you have no idea how to use. Better to invest in good first aid training and make decisions to reduce the risk of injury.
That being said, there are several good lists, one of the most comprehensive coming from Dr Keith Conover http://www.pitt.edu/~kconover/ftp/Everyday%20Emergency%20Kit.pdf
Thats a start... leave out whatever seems like overkill to you, but don't econimise on knowledge!
Speaking of which, that reminds me to get on a refresher course...
Edited: 26/05/2007 at 14:43