How high can a bouldering wall be?
Traditional indoor boulder walls are usually around 15 feet (4.5 meters) tall.
This is not an indication for how long the actual route is, though, since the climbing route may actually be shorter or longer than the wall itself due to the angle of the wall, and to the structure and design of the routes. If the wall is very overhanging, the wall or boulder may be shorter than 15 feet, but the route itself could be longer. Imagine a cave ceiling, it could be rather short in height from the ground, but, the route could span much of the actual cave ceiling, making the route much longer than the actual height of the cave.
Outdoor boulders can be of any height really, though at some point they are considered sport climbing routes, but generally, outdoor boulders can be much taller than 15 feet.