I think on actual physical size EPCOT is the largest, followed by MK, then AK, then DHS... but I'm sure I can be proved wrong! EPCOT is definitely huge - EPCOT can quite easily stand for Every Person Comes Out Tired...
I would say from a ride / show point of view to allow at least 2 days for MK, 2 days for EPCOT, 1 day for AK, and 1 day for DHS. I've actually allowed 2 days per park in my plans, but we're not doing anything else other than Disney parks.
Google Earth is fab for looking around WDW! You really get an idea of the size of the place, and the size of the parks. There are park maps on the web too (although perhaps not the most up to date, but will give you an idea of size when used with Google Earth)
http://www.wdwinfo.com/resortmaps/propertymap.htm
This is good too:
http://disneyworld.disney.go.com/maps/ and those WILL be up to date
Happy planning!