Any idea on the usual capacity of a magnetic napkin card (for example, the one used in an ATM)?
(PS: I tried Google search, but did not get anything satisfactory and put it on)
From http://en.wikipedia.org/wiki/Magnetic_stripe you can see that the map consists of three tracks (as Pax mentioned earlier), and tracks 1 and 3 have a bit density of 210 bps, while track 2 has a density bit 75 bits per inch.
If the strip is 3.25 inches long, it could theoretically store 1607 bits with 682 on each of tracks 1 and 3 and 243 on track 2.
ISO 7813 defines layouts for tracks 1 and 2. Track 3 is covered by ISO 4909.
Track 2 is commonly used for banking operations.
See ISO7813 and ISO4909 for details . Sorry, not Wikipedia for 4909, you will have to buy documents from ISO or your country body.