It has been a while, but I think we limit the navigational database not with absolute mileage but with a certain "maximum allowable difference in latitude and longitude". Therefore the database will be bigger around the equator, smaller towards the poles. The reason we did this is performance. The database has to be accessed all the time, and having too many entries will slow down searches a lot. This does not only affect FMS entries, but we also need to check if a certain fix is on the map to display it... this quickly becomes a burden.
The maximum fuel load is about 16.000 kgs. Under favourable conditions (light aicraft, good winds) this will let the plane fly about 3.300 NM (no reserves). A more realistic range, which the plane will be able to do with reliability is ca. 2200 NM (normal load, average winds, reserves).
Most airlines have databases in the FMS that covers their area of operation - there can be quite a difference in memory available.
I will add the request for a larger database, but we have to check how performance holds up for that...
Jan