Anybody viewing future human evolution from the perspective of advanced western civilisation is unlikely to make confident predictions about brain size continuing to evolve. In any case, all these ways of viewing the matter are far too short- term. Socially important phenomena such as contraception and education exert their influences over the timescale of human historians, over decades and centuries. If we could assume that something like our advanced scientific civilisation was going to last for 1m, or even 100,000, years, it might be worth thinking about the undercurrents of natural-selection pressure in these civilised conditions. In either case, evolutionary extrapolations from present conditions are likely to be highly misleading.
Evolutionists are usually pretty coy about predicting the future.
Our species is a particularly hard one to predict because human culture, at least for the past few thousand years and speeding up all the time, changes in ways that mimic evolutionary change, only thousands to hundreds of thousands of times faster. This is most clearly seen when we look at technical hardware. It is almost a cliche to point out that the wheeled vehicle, the aeroplane, and the electronic computer, to say nothing of more frivolous examples such as dress fashions, evolve in ways strikingly reminiscent of biological evolution. Viewed over the timescale of decades, the average lengths fritter up and down like yo-yos. But mostly, like any other technology, they improve because new inventions build on earlier ones and inventors in any age benefit from the ideas, efforts and experience of their predecessors.