The moon, as seen from Earth, covers an angular diameter of approximately 30 arc minutes (at a distance of ~385 000 km and diameter ~3 476 km, you can work it out for yourself). This is exactly the size at which it appears in Orbiter. There are two possible reasons why it may appear too small to you:
Too large field of view setting. If you want to see the moon and other objects in the simulation window at the same angular size as you would, for example, when looking out of a window, then the field of view (FOV) setting of the simulation must correspond to the viewing geometry. That is, the simulation window size and eye-screen distance. As a typical example, for a 19 inch monitor (4:3 aspect ratio) running Orbiter in full-screen mode, and a viewing distance of 60 cm, the correct (vertical) FOV setting would be 27 degrees. In practice, a much larger value is usually selected, to compensate for missing peripheral vision.
Optical illusion. People perceive the size of the moon to be much larger than it really is, in particular close to the horizon. Several explanations have been brought forward, for example http://www.noao.edu/outreach/nop/nophigh/steve8.html or http://facstaff.uww.edu/mccreadd/. You might want to take a photograph of the moon yourself, with a normal-focus length lens and including some trees or houses for reference. It is going to look smaller than you thought!