The maximum size of a MRDF area can be defined using the COVERED or VIRTUAL MUF startup option or when using the COVERED_ADD command.
However, it can be useful to know how much memory is actually used for each area.
The Memory Resident Data Facility Summary Information (M R D F SUMMARY INFORMATION) report generated at MUF EOJ (or by the ALL_INFO_REPORT command) contains the requested info.
The following query returns useful info too:
SELECT DBID, AREA_NAME, DATASPACE, MEMORY_SIZE,
BLOCKS_MAXIMUM, BLOCKS_CURRENT
FROM SYSADM.MUF_COVEREDVIRTUAL;
See the Memory Resident Data Facility Summary Information report in the documentation for details.