It depends a bit on how long of a trip, and the ability to limp home.

Traditionally my bikes have had very few hex heads. And, the ones it had... 7mm???

Anyway, if you're trying to minimize tools, then look at which part of the multi-tools you're using, and what will leave you dead beside the road if it breaks. I think 4mm and 5mm are the most common sizes that I use. What do the cleats use? Although, it is nice to get all of the common tools together.

I've tossed in a full size chain tool, and since I can never quite find the right spoke wrench, so I carry a 3-way spoke wrench (also good if you need to give assistance to someone).