I've got a group of humans who believe as a central tenet of their religion that humans should be as physically perfect as possible. In pursuit of this goal, they've spent a few hundred years applying an intensive program of genetic engineering and positive eugenics to their population. They've got a few hundred years worth of technological advancements over the modern day, and have a good grasp over the principles of genetic modification, though mostly of the 'modify the existing traits of an organism,' rather than the 'create a new creature in a petri dish' variety.
Their genetic engineering doesn't introduce any new paradigms compared to what we have today, but allows for much greater specificity in its manipulation. New genes can be precisely inserted into specific points of the genome, with manipulation capable of being applied to a single strand of DNA. Knowledge of genetics, of course, has also been significantly developed over the centuries. Geneticists have a good understanding of what most of the individual genes in the human genome do, though research over why a specific set of nucleotides results in a specific phenotype is ongoing.
The eugenics program is state run, and has essentially complete control over all reproduction for this group of humans. They decide who can reproduce and with who, and leave nothing to chance. Embryos are produced in lab facilities where any genetic defects can be eliminated and specific traits can be added.
Their government, which is one and the same as their religious hierarchy, has decided that 'physical perfection' means being as good at combat as possible. Their combat is similar to modern warfare, but with improved weaponry. Powered exoskeletons exist, but are expensive and only extensively used by top level commandos. There may be some hand to hand or melee combat, but as in the modern day, ranged weapons control the battlefield.
Battles in the future mostly take place either on space stations with artificial gravity or in extremely dense urban environments of megastructures. Combat in these environments mostly takes place at fairly close range, with rooms, doors, passageways, and crawl spaces constricting movement and field of view. This, of course is not a hard and fast rule, and a good soldier is expected to be able to fight anywhere, in confined or open spaces and with or without gravity. Philosophically, the society is trying to perfect the human body, so they are uninterested in creating soldiers who are built to be specialists in only one environment.
Within the scope of what is attainable in a few hundred years of genetic engineering, what general body type that would be most beneficial for a soldier? Huge and muscular? Small and lean? Small and stocky? Somewhat average?