>My question was as follows is 12mpix enough on FX to print >what A4 picture or bigger?
However, the physical sensor size is really irrelevant to your question, and this has led to some of the confusion in this thread, I think. As far as possible print sizes go, it's mostly the number of pixels that counts, not the area over which they are spread.
Most people find that a 12MP FX D3 will produce a good print of at least A3 size with no trouble at all. But then, so will a 12MP FX D700, and a 12MP DX D300