Drawing bigger characters in fiction is standard advice. Yet we academic nonfiction writers often forget that we're working with characters, too! In fact, every one of us is a character if we exist in the mind of another. All we are is cartoons in each other's brains, and often inaccurate ones at that. So if you're writing about someone who really lived, the challenges are in many senses the same. The only major difference is that you're dealing with a chronology and a world picture that you can't alter at will (although you can to a certain extent by choosing what to tell, what to leave out, and how to tell it, and therein lies great art), and you are stuck with what the person is known to have done and said... you don't invent it. But after that initial difference, the crafts become so similar.
My book is primarily about a woman, but I'm also writing a bit in one chapter about her father, who died in the late 1600s. He appears so faint to us now, hundreds of years away, so I'm using the tools of fiction to magnify him. It's like literary amplification -- adding color and sound where there was none previously to bring a once-vivid character back to life. Is it fiction? No -- he really lived, and I'm doing my best to work with the facts of his life as we know them. But it will benefit from many of the tools that make good fiction so fun to read.