Are you trying to do this with the 50mm lens? If so, you need to try the 105mm lens.
And with 6x6 negatives and the 105mm lens the usual problem is with the image being too small, not too big.
If it is the 105mm lens you are using, is your problem related to the amount of bellows travel available when focusing? I could be wrong, but I seem to recall that the condenser adjustment available on the 23CII can affect that.