Given the powers of the federal government, and the fact that DC was even bigger than it is now (virginia's land was returned) why did the creators of the US government's seat feel that they needed 100 sq. miles of land? Even with possible growth, why did the organizers feel the need for so much space to handle the relatively small operation of the time of the federal govt?
There seem to be two things going on, from perusing the Congressional records and the Washington Papers at LOC.
They only wanted to do this once, and were very concerned about getting a good amount of land for their money. Remember that you didn't have refrigeration or canning in 1790, so you needed to be able to have dairies and stables etc. etc. Travel by water was key to the location of the city, so you also needed to be able to support shipbuilding and at least one port.
The federal treasury was small and they did not particularly want to go into debt for this. So they intended to sell parcels of land in the city to fund the building projects for the various federal buildings (Here is a broadside from the Washington Papers at the LOC giving info and terms of that http://memory.loc.gov/cgi-bin/query/P?mgw:2:./temp/~ammem_aujF::)
You can peruse a lot of source materials related to the founding of DC at the Library of Congress site here: http://www.loc.gov/rr/program/bib/ourdocs/Residence.html