Training for the corpus callosum? Since reading is done in the left hemisphere of the brain, does reading text in the left visual field train the corpus callosum? For instance, I am interested in knowing whether training may lead to an increased size of the corpus callosum (e.g. more fibers), and whether this translates into some cognitive benefit (e.g. faster response times or comprehension of text in the left visual field).

I think so because I recently saw a document where it was reported that a guy who had his corpus callosum damaged by a bolt couldn't read words on the left anymore. The reason for that is/was that words in the left visual field go to the right hemisphere of the brain. To be read the information is then transported via the corpus callosum to the left hemisphere.
It's been known that Increased Corpus Callosum size was found in musicians who began music training before age 7 and that Training working memory leads to growth in Corpus Callosum.  As for how to train working memory, that's a diverse subject.  There have been a lot of experiments involving Training working memory for ADHD patients, however a meta-analysis has revealed that most of the training from these ADHD-centered programs only leads to short-term gains, not long-term or generalisable gains.

To speculate some, I think taking on new tasks and challenging yourself repeatedly in a wide context (as opposed to isolated tasks "designed" for working memory) is your best bet.  Learn a new instrument, learn to program in a new language, learn new regions (spatial mapping is associated at least with hippocampal cell growth as long as you use spatial strategies, not procedural ones).