On the issue of the height of players I do think there may have been a very small drift upwards since the 50s and 60s, but it is very small, and much less than some people think.
There is no doubt that people generally have got bigger due to diet (I hesitate to say better diet because we know the issues surrounding diet), but of course it is more complicated than that. For starters it seems that people have got wider as much as as they have got taller - I love the representation of the future population in WALL-E as round blobs! And bigger does not mean better, since a recent scientific survey showed the current generation of schoolchildren in the West is actually slower than their parents were at the same age, due to lack of exercise.
However even without the general increase in height I think the modern game would have benefitted the pre-existing population of tall players for two technical reasons. The first is the move to more consistent, higher bouncing surfaces. That makes life much easier for a taller player because the ball is now coming into their preferred hitting zone. Life is much tough when the ball is bouncing variably, and coming through around your ankles. That is why players in the past were generally built more to be able to react quicker, but with less power than nowadays.
The second reason is the increasing ability to take breaks in matches, which can be either the official changeover breaks and the unofficial towelling breaks. Both these factors make like easier for heavier players, and taller usually means heavier. There is a reason marathon runners are small wiry men.
So increased population height, higher truer bounces, more rest time during matches all help make the game easier for taller players, and we might expect to have seen a significant increase in height for the top players.
But then things start filtering all those taller players. Even before tennis starts filtering, other sports start filtering too, because they have improved their surfaces too, to make life easier for taller athletes. Soccer in particular has improved pitches out of all recognition, so there are no longer boggy, slippery patches favouring small, nimble, athletes. American football looks to me to have done the same, and hockey has gone the astroturf route. So they now recruit many of the good tall athletes which they might not have done in the past.
Soccer, in fact, seems to me have increased in height significantly more than tennis, and that is because tennis also has the technical factor. Taller = more leverage = more stress = more injuries (ref Del Potro). The other technical factor is the small size of the tennis court and the nature of the game, which puts a premium on rapid acceleration from a slow or standing start. This starts working against taller players, so despite the increased population of tall players, the optimum size still is around 1.85m (6'1").
If all this is true then I would expect the height of tennis players in 1965 and 2013 to be like two triangles. At the bottom, the millions of recreational players, the triangles are wide, because size matters less than motivation, people play for enjoyment. The triangle is wider in 2013 because of more taller players. But as you rise up the triangles towards the top players, then the tennis filter kicks in and people who are too small or too tall drop away because they cannot compete with those who are better sized. Both triangles the one in 1965 and the one in 2013 narrow, until at the very top the triangles are at their narrowest, and the difference between 1965 and 2013 has narrowed significantly.
In fact I would predict that the differences would be less for the top 4 players than for the top 8. So let's do that for the top 4 seeds in 1965 and 2013. Using the heights I gave earlier
1965 Average height of top 4 seeds = 1.90m
2013 Average height of top 4 seeds = 1.85m (!)
So the top 4 in 2013 are actually shorter than in 1965! The reason is obviously because of Ferrer, but even if I swap Nadal for Ferrer
2013 Average height of top 4 seeds = 1.87m
still shorter.
Of course we are considering very small populations, so you can't read too much into any one comparison, but it does show again that 'modern players are bigger therefore better than shorter players from the past' is not borne out by the facts.
In fact if I had three players heights 1.75m, 1.85m, and 1.95m in height and they were all exactly equal in ability and I had to choose one to play tennis for my life, I would take the 1.85m player whether they were playing in 1913, 1963, or 2013.
* As a footnote I have ignored the extra factor that the shorter player can have - that extra determination to prove themselves aka the Napoleon syndrome. Ignore that at your peril!
