According to this modder's resource plugin for Fallout 3, the NE corner cell has the coordinates (25, 34) and the SW corner is (-34, -27). That gives a total width of 60 cells and a total height of 62 cells, for a total of 3,720 cells. That's including the unplayable areas outside the rectangle borders, though.
Counting only the width and height of the playable area, it appears to be 51 cells at the widest point and 51 high, or approximately 2,601 cells. Even then those aren't all player-accessible, as the invisible-wall border is irregularly-shaped, but that's close enough for comparison to other Bethsoft games.
Fallout: New Vegas
Unfortunately I couldn't find a modders' map for New Vegas, which surprised me – I guess the tales of its editor being riddled with bugs and preventing the modding scene from really taking off have some merit to them.
Instead, I set out into the Mojave Wasteland to do some direct surveying. (I'll update this when I return, if the geckos don't get me first.)
The geckos didn't get me, but only because I used god mode. It turns out that the end of the world did some damage to the surveyor's tools though: the
toggleDebugText console command doesn't work in New Vegas, so we're going to have to rely on object coordinates and do some conversion.
Revealing all map-markers using the console, I fast-travelled to the westernmost map marker (Charleston Cave). Moving further west (while no-clipping) to the furthest-west placed object (a bunch of rocks above Jacobstown) I took a reading:
> player.getpos x
Dividing by the width of one cell (4096 position coordinates) and rounding down that gives us the cell's approximate X coordinate: -29.
Repeating this for x or y at various other locations (north of Nellis Array:
161736.50, east of Legate's Camp:
111398.05, southwest of Crescent Canyon East:
-127206.66), the survey gives a maximum width of 57 cells and a maximum height of 71 cells, for a very, very rough overestimate of a rectangle of 4,047 cells inside of which the invisible-wall border is irregularly inscribed.