LimeWire itself doesn't take up much space about 18.5MB on Windows, I am not sure about MAC but it couldn't be much more. Now the files that you download with LimeWire could be a different story. The average music file is about 4.5MB so 100 would be 45MB 1000 would be 4.5GB. Movie Files could be anywhere from 50MB to 2GB. And software and Games can be anywhere from 5MB to 5GB so you see it all depends on how you use LimeWire as to how much space it takes.
As far as LimeWire Being safe it is only as safe as the person using it. There are Virus and spyware files on the Gnutella net but if you do a little reading and educate yourself before using it is very safe. And having a MAC will help in the Virus department.
Hope this helps below are a couple of links you may want to read.
http://www.gnutellaforums.com/showthread.php?t=58603 http://www.gnutellaforums.com/showthread.php?t=56692