Andrew Sarangan wrote:
I am not sure what kind of separation is used by ATC
when in a hold. It can't be that close because the turn radius depends
on the aircraft speed. During the outbound turn from the holding fix, a
Citation will go past the holding fix much farther than a 172.

Of course. All this is covered in TERPS

http://av-info.faa.gov/terps/directives%20page.htm

If you read the chapter on holds, you will know more than you ever
wanted to about how these things are figured out. Faster airplanes are
allotted more protected airspace.