If the 'Generate Forests' option is selected, then clusters of trees will be generated in addition to the individual trees that are placed on the map.
Note that until the first prisoner arrives fog of war will not go into effect.
(Note that the Accountant is able to research Land Expansion later in the game.) This does not directly affect the difficulty of the game, as the initial resources given to the player remain the same, but it does affect your computers performance. A small prison is 100x80 squares, a medium prison is 150x120 squares, and a large prison is 200x160 squares, where each square roughly corresponds to a square meter.