Well I'd say at least 20MB but then of course you need space for attachments and also the database space, so depending on your expected activity I'd say at least 50MB.
(I have 15 Mb left at this moment ... - so what's wrong?)
I know that I need much more free space to run the script. Right now I want to see that everything going fine - I can remove a few things (old forum, etc.) to get more space, but do I really need so much as 50 Mb?
Out of memory isn't to do with disk space - it is your server memory. Either your server is lacking memory or there is a problem with perl causing the memory error.