Neanderthals are shorter and stockier than modern humans, but where comparable in height to the sympatric Homo sapiens of the time. One can only assume that if a modern Neanderthal was to be found and raised on a modern, Western diet, they would be significantly larger and healthier than their primitive cousins, similar to the way that new generations of H. sapiens ''natives" are larger and healthier when fed healthy, Western food than are native populations eating roots and bugs and whatnot.
It is entirely plausible to speculate that a modern Neanderthal, with all of the medical and nutritional benefits available today, would be a giant in comparison to modern humans. They would also likely be smarter; Homo neanderthalensis had the largest braincase of any hominid, to include modern man.
It is hypothesized that they went extinct due to the much higher metabolic needs of their large body and brain. Basically, Homo sapiens could do the same thing as the Neanderthals in a cheaper-to-operate body. Essentially, it was a prime example of competitive exclusion.