Although FreeBSD runs nicely enough in 512 MB, we feel that 1 GB really represents the sweet spot.
In the event that you want to increase or decrease the memory dedicated to VirtualBSD, edit VirtualBSD 9.0.vmx and look for the line that reads:
memsize = "1024"
and change it to the desired new size, say 512, like this:
memsize = "512"
Warning 1: Make sure you reserve at least 512 MB for VirtualBSD!
Warning 2: If, on the other hand, you would like to reserve more than 1 GB, make sure there is enough RAM left for the host computer. Requirements are hard to predict in advance but, at the very least, there should be 512 MB available for the host OS.
Save, exit the text edtor and start VirtualBSD again.