The amount of memory available for record-based data storage varies from device to device. The MIDP specification requires devices to reserve at least 8K of non-volatile memory for persistent data storage. The specification does not place any limits on the size of an individual record, but space constraints will vary from device to device. RMS provides methods for determining the size of an individual record, the total size of a record store, and how much memory for data storage remains. Remember that persistent memory is a shared, scarce resource, so be frugal in its use.
Any MIDlet suite that uses RMS should specify the minimum number of bytes of data storage it requires, by setting the MIDlet-Data-Size attribute, in both the JAR manifest and the application descriptor. Do not set this value any larger than absolutely necessary, as the device may refuse to install a MIDlet suite whose data storage requirements exceed the space available. If the attribute is missing, the device assumes the MIDlet suite requires no space for data storage.
In practice, most devices allow applications to exceed their stated space requirements, but do not depend on this behavior.
Note that some MIDP implementations require you to define additional attributes related to storage requirements -- check device documentation for details.