This is a great point of reference. However (and this is purely personal preference), if you want to save space, you can skip the odd zoom levels and only use the even ones. You don't lose a lot of detail, and Maemo Mapper will still work at the odd zoom levels - it will just do a manual zoom of the next-higher level.

For example, the following sample setup for a city on the Eastern U.S. Seaboard should easily fit on a 1GB memory card (rough-order-of-magnitude square kilometers in parentheses):

Level 0: 10-km square around your house (100).
Level 2: Metro area of nearest city (1000).
Level 4: Metro + Suburbs of nearest city (10,000).
Level 6: Area surrounding the nearest few big cities (100,000).
Level 8: Eastern Seaboard, or maybe even everything east of the mississippi (1,000,000).
Level 10: Entire United States (10,000,000).
Level 12, 14, 16: Entire World (purely for showing off, e.g. "I can zoom out and see the whole world!) (500,000,000).

If you want to put more effort into it, you can download Level 6 detail for all major cities without too much trouble. This would be enough to drive through or around those cities using major highways. Rural areas, though, tend to have maps that compress better (more open space), so it's not a complete loss to keep maps of rural areas.

Zoom levels are roughly quadratically related to disk space. Each zoom level is exactly half the scale of the next-higher-resolution zoom level. 1 is half the scale of 0; 2 is half the scale of 1; etc. For a rectangular area, then, zoom level 1 takes up about 1/4 as much space as zoom level 0. Skipping every other zoom level saves you about 50%, as does removing your smallest-scale zoom level (assuming you have all the other zoom levels). Doing both doubles the savings! In other words:

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16
2, 4, 6, 8, 10, 12, 14, 16
4, 6, 8, 10, 12, 14, 16

Each of those lines, except for the last line, takes up about half as much disk space as the previous line. The last line takes up about 1/4 as much disk space as the second-to-last line.

One last thing to consider: using Maemo Mapper with FAT16 will be very wasteful due to the internal fragmentation. Block sizes in FAT16 are 16 kB, so even the small maps (the smallest are about 100 bytes) will take up 16 kB. With FAT32, you can set the block size to as low as 512 bytes, which is fine for typically sized memory cards (it only becomes a problem for large hard drives) and will save you a lot of space.

Maybe someday I'll consider replacing the file-system back end with a sqlite database, but something tells me that will be very CPU-inefficient. Plus it would completely break everyone's map caches!