At the local steak and shake I came across an aluminum tin used for holding gift cards. It felt almost like the perfect size for my purposes.
It's the same length and width as a tobacco tin (5" x 3.25"), but not as thick, it has the same depth as an altoids tin.
For those who use tins vs. other carry methods, a tobacco tin sized kit offers several benefits over an altoids tin; it's wide enough that you include a real pocket chainsaw that lies flat on the bottom, which is much less likely to break than a wire saw.
Additionally, it's just wide enough to fit in a whirlpak style water container, allowing you a true 1 liter capability as opposed to other carriers like breastmilk bags where you might have to split up a water purification tablet.
(example)
However, it's so thick that even though it'll fit into a cargo pocket it's not the most comfortable thing to have banging into your leg while you're moving.
The steak and shake tin seems to be an excellent compromise in size and capability and is more comfortable to carry in a cargo pocket.
Just thought I'd share!