In good sized, well mainained tanks, the regular Congo Tetra can grow to 4"/100mm although frequently less. The Yellow Congo Tetra is also a large tetra but slightly smaller then the regular. The Yellow is also a bit more demanding of water conditions and can be difficult to settle, although once over any initial problems tend to be okay.
At the size your are, I would expect to see some of the fin extensions. Can you see them, i.e. are they just not apparent in the picture?