You make a space bigger on the inside than on the outside. You double either the length, width, or height dimension of a bounded space (any space with some sort of floor, ceiling, and four walls), if the additional space does not exceed the maximum area allowed by the power. Though one internal dimension doubles, the external size of the space is not affected. You can only double the length, width, or height of a space that is already at least 5 feet square. You cannot double a dimension of a space that is already doubled or otherwise psionically or magically maintained (such as the interior of a bag of holding).
For instance, an 8th-level manifester could double the width of a space 15 feet long, 10 feet tall, and 10 feet wide to make its new interior dimensions 15 feet long, 10 feet tall, and 20 feet wide—in all, the manifester added only three 10-foot cubes of space, well within the maximum allowable area. If the space had been 25 feet wide, the manifester couldn't have doubled the width.
250 XP per additional 10-foot cube of space created.