How far was the surface of last scattering at the moment of recombination? If I understand correctly, then the surface of last scattering (the last particles off of which CMB photons scattered before traveling for 13 billion years and reaching us) is currently at a distance of around 42 billion lightyears from us, just before the particle horizon. 
How far away was this surface at the moment of recombination? 
Also, when we calculate the curvature of the universe by comparing standard rulers on the CMB to their measured angular sizes, which distance do we use as the "distance to object"? Do we use the distance described in this question, or the distance to the particle horizon, or some other distance?
 A: The distance to the surface of last scattering was 0 at the moment of recombination.
The distance of last scattering (the raius of the surface of last scattering) at any time is the distance from which the recombination radiation (which today is the CMB) can have traveled since the time of recombination. Since that time was zero at the time of recombination, that distance must also have been zero.

EDIT:
If this question is to be interpreted as: "If we take the current SLS distance and shrink it along with the Universe as we go back in time, what does that distance become then?", then the calculation is pretty simple. If we assume a standard cosmology with $H_0=70$ km/s and $\Omega_{m,0} = 0.3$, and a redshift of 1100 for Recombination, then we get a currrent, comoving distance to the surface of last scattering of $\approx 45$ Gigalightyear, so we simply divide that number by 1100 to get $\approx 41$ million light years.
But that is not really a physically meaningful distance*.
*: Although maybe it is, see @benrg's comment below.
A: I hope this clarifies things.
If recombination were been an isolated event, i.e. something that happened in a certain place with some finite spatial extent, then it would make sense to ask how far it was from us when t=380kyrs.
However recombination was not an isolated event, it happened everywhere in the universe at the same time, light scattered for the last time in all directions. From our location in earth CMB photons were emitted as well (that's why our distance to the surface of last scattering was cero at that time, because we emitted CMB radiation) but those photons are now very far away from us.
The photons that we do see are those that were far enough at the moment of last scattering so that their trajectories are meeting us today. That far enough distance is the radius of the surface of last scattering, because in every direction, at that radius, we see this last scattered photons.
If another planet looks far enough they will also see their own last scattering surface, with the same radius  as ours. They will also observe the CMB because it happened everywhere.
