Vista - Virtual Server 2005 R2 - 2 questions (VHD's and MAC ID's)

I have 2 questions about running Virtual Server 2005 R2.

1.) Will the size of the VHD file effect server performance in any way? Will
an 80GB fixed size VHD file perform better then a 500GB fixed size VHD file?
Taking into account Virtual Server 2005 will be installed onto a low hardware
spec machine. I have a 640GB hard drive I will be using just for my VHD file.
I don't plan in the near future to ever use more then 40GB on this server.
But I figure if there's no downside to creating a 500GB VHD file, I might as
well allow the server to use most of the hard drive right?

2.) When editing a virtual machine's network adapter properties, it defaults
to a dynamic MAC address. But there is the option to create a static MAC
address. The dynamic setting sends the same MAC address the host server
(running Virtual Server 2005) is using. So within my router (which does my
DHCP), I assign the same MAC address to two different IP address'. On each
server I then tell it which IP address it should be using. This appears to
work without any problems. But I'm wondering if it's really a good idea to
use the same MAC address, or should I just make up a fake static MAC address?
I would think a fake static MAC address would be best because it would
potentially cause less confusion in the network. Anyone have any input on
this?