Good tiny device for storing/summing up a few lists of numbers throughout the day?

[Jim Horn]

If you'd like something for this specific task, rather than incorporating it in a cell phone, you might consider a Palm Tungsten or similar. They've fallen out of favor so can be found very inexpensively (I have nearly 100 that were thrown out at a previous employer - anybody want some?). They have an easy to read screen, a variety of input options (I found Graffiti to be very easy to use) and are roughly half the size of a 48/49/50. There is a huge variety of free software available as well. I used mine heavily (including Free42) until my non-smart phone died and was replaced by an Android one four years ago. It still has software I've not found for any other device.