Without any hard numbers this is nearly impossible to quantify. Suffice it to say a tractor beam should be powerful enough to keep a sublight vessel immobile. IIRC I believe one of Voyager's shuttles was caught in a tractor beam and they tried to go to warp and failed, though it was probably because they couldn't form a warp field inside a tractor beam (Episode citation needed).
The depiction of Star Treks technology is predicated upon the assumption that there is much about particle physics that we currently do not understand. Treknobabble is rife with references to various kinds of particles the manipulation of which is the mechanism of many technologies. Even today, particle physics postulates the existence of particles that currently cannot be proved to exist--which, if their existence could be proven and their motion understood, would, theoretically, lead to the development of some of the technologies, and abilities thereby, that are depicted in Star Trek ''ProfessorTrek 01:20, August 8, 2011 (UTC)
Based on this theory, the tractor beam probably works by manipulating gravitons to effect and affect the movement of, or directly manipulating the movement of, quasiparticles. Its intensity would be subject to the inverse-square law ProfessorTrek 01:28, August 8, 2011 (UTC)