The Pandora has 512mb of internal memory on the NAND.
A clean installed unit will have "about 80mb left". Not being careful (especially by installing too many things with OPKG, or using the version of ClawsMail included in the OS) could mean that you'll run out of room and need to reflash your Pandora.
into the terminal to check how much space you have left on the NAND. Type
du -h | more
into the terminal to find cache folders and how many megabytes they are using (then delete what is in them to free up space).
A more advanced approach will be:
du -x /|sort -n|tail -20
which will show you the top 20 in directory sizes on the NAND only.