haonguyen91,
You need to tell PhotoScan something about your real-world otherwise it can only work in abstract co-ordinates. The same pair of images could be of an object 10 cm away with the cameras 5cm apart - or objects 100m away and cameras 50m apart. The only way you can get measurements out of any photogrammetry system is by putting reference information in - that might be something like a couple of measurements between points if it is a small object, or a set of survey ground control points if it is an aerial survey; or, if you have sufficiently accurate camera positions, you may be able to infer scale from them.
No reference information => no real-world dimensions.
DM