So far, according to media reports, the business-end of Deep Impact that has been launched to hit comet Tempel-1 is the size of
- A wine cask (NASA)
- A washing machine (The Guardian, CNNi, News.bbc.co.uk*)
- An oil-drum (BBC Radio 4)
I will keep this list updated with more popular-media-sizing-analogs for space probes (half-a dolphin!).
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* In keeping with other white-goods in North America, are US washing-machines bigger than their UK equivalents?
Petafloptimism 
Yes, US washing machines are bigger. They tend to be top-loading and faster, compared to the smaller, slower, more efficient front loaders favoured by Europeans.
See http://en.wikipedia.org/wiki/Washing_machine#Modern_machines for a comparison of front and top loading machines.
Today, the Metro also said it was washing machine sized, but last week they said it was coffee table sized.
Some other randomly sourced comparisons:
“wine barrel-sized”
http://www.detnews.com/2005/nation/0506/27/A04-228771.htm
“coffee-table sized”
http://www.telegraphindia.com/1050703/asp/foreign/story_4944245.asp
“armchair-sized”
http://venues.surfwax.com/files/College_Park.html
“TV-sized”
http://www.msnbc.msn.com/id/6817717/
“refrigerator-sized”
http://www.middlebury.edu/administration/middmag/current/pursuits/
And have you also that other (big, flat) things are often compared to the size of Wales?
“Like firing a fridge at a cliff-face” on Channel Four news this evening.
Yes, but how many hogs-heads is it ?
NY Times reports nothing on the size of the thing, but does claim that the crater is either “as large as a sports stadium or as small as a house”.
I would be very satisfied if the world could actually standardize on dolphins as the SI unit of size/volume for scientific analogs.