No one defends and stands by the decision of casting Ben Affleck as Batman/Bruce Wayne more than Zack Snyder. The director of Batman v Superman: Dawn of Justice has specific reasons for casting Affleck aside from his portrayal. His look and size also factored in quite largely.
Snyder went on to say:
"I never liked a small Batman. Ben's all of 6-foot-4, and in the boots, he's like 6-foot-6"
Which is a fair statement as Batman has always been quite large in stature, except in all of the past live-action portrayals. They were all small to a certain extent, relying on the suit to help increase their size. Meanwhile the Batfleck is looking very reminiscent of his comic counterpart, especially that from the 'The Dark Knight Returns' graphic novel.
Along with his size, Affleck matches the old, grizzled and experienced aspects of Frank Miller's Batman. Whether or not he will actually be good in the role is left to be said. But so far, he is looking pretty damn good.
Fearing the actions of a god-like super hero left unchecked, Gotham City’s own formidable, forceful vigilante takes on Metropolis’s most revered, modern-day savior, while the world wrestles with what sort of hero it really needs. And with Batman and Superman at war with one another, a new threat quickly arises, putting mankind in greater danger than it’s ever known before.
'Batman vs Superman: Dawn of Justice' opens on March 25th.