Q:

What is the smallest body in which a sling shot maneuver can be performed?

This question asks about the smallest sling shot maneuver performed. What was the smallest intentional, acknowledged slingshot maneuver?
I'm asking how small of an object be to perform a sling shot maneuver around it?
Update
I didn't think of Magnestars and micro-blackholes when I wrote this question but the replies are great.
Within our solar system I would guess that an object lacking an atmosphere a spacecraft could get closer to the surface of it during the sling shot.

A:

How small do you want to get? $F=G{Mm \over r^2}$ applies regardless of size. If you remove enough disturbances from other bodies you can get two neutrons to orbit a common barycenter on gravity alone - or send them against each other on a near-miss trajectory and they'll pass influencing each other gravitationally in essence performing a slingshot against each other. 
That's considering mass. Considering size as radius - a singularity is dimensionless, zero size, and can easily slingshot planets or smaller stars... but if you're going to slingshot against one, better stay well clear of the event horizon, which may span many kilometers past the dimensionless singularity point.

A:

We've done a slingshot maneuver with the Moon.  That's essentially what Apollo 13's free return trajectory was when the spacecraft became crippled and had to be returned to Earth.

I would like to address some comments this answer has drawn.  First, some have said that this dies not answer the question of what the smallest object suitable for a slingshot is.  But that question has no clear answer because technically any gravitational deflection that does not result in capture is a slingshot.  [This reference] describes a (very low angle) slingshot-type maneuver past the Martian moon Phobos used to nail down it's mass and density (thus, it's porosity).  Thus the above is intended as a  practical example of a much more significant slingshot involving an object smaller than the planets.

