Fallout 3 uses a coordinate system with units which, as in Morrowind and Oblivion, are 21.3 'units' to a foot, or 7 units to 10 centimeters (or to put it another way 64 units per yard [~70 units per meter]). Note that the Game Settings fBSUnitsPerFoot and fMagicUnitsPerFoot define 22.0 units per foot by default, but these are likely vestigial remnants of Oblivion's area-effect spells.
Another way of approximation is to assume that the average height of a human is 128 'units', or six feet. 128 divided by 6 is 21+(1/3) (twenty-one and a third). Round this down, and 21 units per foot gives an average height of about 6' 1.14". This seems to be a reasonable approximation.
Cells and Quads
The base of Fallout 3's landscape system is an exterior cell which is 4096 x 4096 units or 192 x 192 feet or 58.5 x 58.5 meters. Cells are organized into quads which are 32 x 32 cells or 1.164 x 1.164 miles or 1.872 x 1.872 km. (See the image below, which is taken from the CS wiki.) The drawing canvas in the heightmap editor shows an area equivalent to 4 quads (2 x 2). The upper limit of supported heightmap area is shown in the overview window of the heightmap editor, which is 16 x 16 quads or 18.624 x 18.624 miles. To put this in perspective, the Wasteland worldspace occupies a total area of 6 x 6 quads or 6.984 miles x 6.984 miles although the usable area is only about 50 x 50 cells or a little over 1.8 miles across.
Heightmap Units vs. In-game Units
When importing height values for terrain into the GECK, 1 person height is only 64 units. Which equates to a smidge under 35 units per metre (32 units per yard). Also, the game seems to round height values down to the nearest 4 units, so this gives a vertical resolution of 5.7cm (2.25 inches). But when you load your terrain into the game, it seems to scale the height by 2 so that 64 vertical units equals 1 yard again! Keep this in mind as you are editing the terrain in the landscape editor: slopes are steeper in-game than they appear to be when editing terrain in the editor, so be gentle and spend a bit of extra time smoothing your terrain mesh.