I tried posting this reply previously and it didn't show; sorry if it shows twice. Check here for some information:
Are you a developer who uses NSIS to distribute your application? Are you a Winamp plug-in developer who wants to use NSIS to distribute your plug-in? Have suggestions for other people like you? This is the place.
I did a search for "space required" in the NSIS discussion and found quite a bit of help so if this is does not answer your question, try that search and you may have some luck.