To display the free space of a file system just enter the command:

df -h

You will get a report like this below (the option -h prints sizes in human readable form):

Filesystem                Size      Used Available Use% Mounted on
/dev/ram0                 7.4M      5.8M      1.6M  78% /
/dev/mtdblock5           16.0M      6.1M      9.9M  38% /ffx
/dev/mtdblock6            8.0M    836.0k      7.2M  10% /ffxusr
/dev/ram1                15.5M      1.0k     15.5M   0% /tmp

Keep in mind that the JFFS2 flash file systems mounted on /ffx and /ffxusr work with data compression. An average compression rate is about 50% for binary application data, i.e. 1MByte of free space on this file systems means that you may copy approximately 2MByte of user data to that file system. The actual compression rate clearly depends on the type of data.