The absolute smallest usable capturing device would have to be a cellphone... IF you need to capture audio, video, photos, and also notes. There is no combination of paper + camera + audio recorder that could compare.
However, for low-tech, you have a tradeoff between tiny dimensions and usability. Blank business cards are pretty small, but that size or smaller can start to look silly in many contexts. As the paper gets smaller, the usable percentage of real estate gets smaller and smaller due to margins.
I settled on 3x5 cards. Even though they are larger (length x width) than, say, business cards or tiny Post-its, they are less bulky for the amount of space they provide. In other words, instead of carrying 10 blank business cards, I can carry 3 blank index cards--slim enough to fit in any pocket.
Something truly tiny, like the mini Post-it notes, look absurd in many contexts anyway.
I guess you could get smaller and usable by just carrying a miniature Sharpie marker and writing on your arm, but I'm guessing you meant high-tech or low-tech, not "caveman-tech".