One easy way to experiment is to use winmapper (in Windows) or Maemo-Mapper-on-the-Maemo-SDK (in Linux) to do the downloading on a desktop machine (with a fast network connection). Once you get the sizes right (taking into account internal fragmentation, which might be different between your desktop's and Nokia device's file systems), you can then simply transfer the maps to your memory card and use them as-is in Maemo Mapper.