A map's width and height are given in 'tiles', and the last row and column are not displayed for technical reasons. A tile is 2x2 'cells', these are one unit square. So a 64 * 64 map is effectively 126 * 126 cells, and hence 126 * 126 units across.
Note: despite the last row and column of tiles not existing in the game, they are shown in the map editor.