The two values I've seen are 25 sq ft and 20 sq ft per cabinet... If you are doing two-post open racks I'm sure that would be lower, but spacing is still similar as you need to account for manipulating servers and cooling rows etc.
Anthony M. Faoro II - tmf [at] adtaq.com - (425) 444-8787 ex 7000 Seattle Colocation @ Adtaq Internet | Seattle, WA | Be Happy! AIM TonyAdtaq | GTalk tmf [at] adtaq.com
Most 4 post racks are around 22" wide (say 24" for ease of calculations), and around 2.5' (30") deep. Considering you need roughly 3' both in front, and behind a row of racks, you can expect each rack to require roughly 24" * (36" + 30" + 36"), or around 17sqft, cabinets should count on at least 18sqft/ea.
An additional 2-3' walkway should be added somewhere in the row (to get from front/back, unless you can get multiple doors added to your cage), additional rows can share the back (hot) row, so each additional row that can be added will increase the space utilization. Generally speaking, the shared back row will even out the additional space that must be left for a walkway (assume you'll loose around 27sqft for a 3' wide walkway between the front/back of the row).
With only 20A per rack or cabinet, there shouldn't be any serious concerns for cooling that type of setup.
3' walkways are required in some places (S&D claims its to maintain compliance with a disabled workers act for example), but not all, its simply a typical/suggested value. You can go with less, but maneuvering may become a problem which cannot be easily resolved down the road (esp if you have servers with tricky rail mechanisms, cable ties/rings/loops, etc.)
Generally rackmounts cool from front to back. so in front of every rack you should request to have perforated panel put in place. So you basically have a cold isle, hot isle, cold isle. This allows servers to cool and hot air from 2 rows to be pushed into 1 isle.