In general, you will need a good 'stage' condenser-based mic rather than a dynamic/vocal one to cover such an area - and even then you'd probably need more than one, and a mixing desk that can supply 'phantom power' to the mic/s. You could then bring the mixer out to the line or aux in on a PC running skype. If the target PC only has a mic in then you may need to do some signal matching as a line out signal from a mixer will swamp a mic in.
Beyond that, it depends on what you plan to achieve - you might need wall or ceiling mounts, for example, as I can't see you wanting floor stands that everyone trips over.
Here's a good overview of microphone types: http://homerecording.about.com/od/microphones101/a/mic_types.htm