Remember that there is a whole more that goes into the size of the ‘ideal’ warrior then just how big of a sword they can swing, especially when they are part of an invasionary force (and one that requires the majority of the force to move itself, on foot). First, there is the issue of food, armies eat a lot (understatement of the year), and bigger people (generally) eat more. Second, and this is especially true with formation fighting (like the Romans took part in at the time), when adjacent soldiers are of different size, there is the probability that they don’t work all the well together. As such, having similar sized people makes things easier. For example, I am 6’, if the man to my right is only 5’6", he may not be able to adequately protest my head with his sheild (as was common practice in phalynx fighting), plus, if we were breaching a door, I would have to crouch to maintain equal sheild height, if I were tasked with overhead cover.
Additionally, having a bunch of people of similar size makes outfitting them much easier (my platoon had two guys 6’5" and 300+#, do you have any idea how much effort it took supply to find the proper sized equipment (uniform, boots, body armor). Third, there is a rapid diminishing return on size in infantrymen. The extra (absolute) strength gained by the weight is 1. offset by technology (tools, such as swords, spears, battering rams, etc), 2. all that extra weight has to carried around (remember, infantry walks everywhere they go, unless they have a Bradley <which the Romans didn’t>) 3. If everyone is given, roughly, the same food ration, the bigger guys are likely to fatigue even more quickly, due to undernourishment, 4. Simple numbers overwhelm any advantage from one man’s size (ever watched ants tear apart a spider?).
Along those same lines, the equipment Roman foot soldiers had to carry was not very heavy, and sword, sheild, some water, some dried meat and some almonds (after the invasion of the Andulusian penisula, at least). And, you would be amazed at what relative small people can carry for long distances, go work with either the Korean or Bolivian armies for a few weeks, or just visit Guam.
So, when it is all said, I don’t see ANY advantage to having larg(er) foot soldiers for an offensive army. Note: I am not commenting on gladiators, palace guards, catapult operators (who, undoubtedly had beasts of burden for their equipment), etc. For a real life example of this, check out some photos of any light infantry (Rangers are the best example) unit; all fit, muscular (by average American standards, but not necessarily by T-Nation’s), and relatively skinny (at 6’ they would be between 170-185#, 200# if they have been in for 6-10 years, maybe). Oh, and the average combat load carried by todays US Infantryman is in the neighborhood of 80# (more then any infantrymen has ever had to carry), and their marching loads can be over 120#.
To sum it up, the ‘ideal’ foot soldier would be, 1. big enough to carry his gear and do his job effectively (knowing that no one soldier is expected to be Superman), 2. small enough to not be a waste of resources, and 3. just about the same size as everyone else in his unit.