In the Scenario Editor open Edit Scenario (the pencil & ruler graphic near the upper right corner) > Park tab > Edit park dimension and guest spawn points button > in the Adjust Park Boundary panel increase the Height to its max (625 meters, if you put in anything higher it'll default to that). That having been said, the game (Windows version, I don't know about Mac or consoles) has two physical restrictions on coaster height, one on the coaster (different depending on what type it is) and one on the park. The game will detect potentially lethal conditions (usually high-Gs) during testing, and the guest brain knows the testing results, so most guests will refuse to ride a coaster that's too dangerous.
