I had a customer this summer who has a new home in a new subdivision
near me in rural Oklahoma. His property includes about 1 acre in the
back that is undisturbed. He has asked me whether this area is
sufficient for a natural area, and I told him I wasn't sure whether the
pressures of development in the area would overwhelm his native
flora/fauna. Are there guidelines for this? How should he proceed?

I would suggest doing a little reading into permaculture, with an
emphasis on native plants and micro-niches. An example of this line of
thought would be: "what different kinds of habitats were there in the
area of the subdivision, and which micro-habitats can I replicate and
maintain access for native species to."

Nature's processes get to work in something as small as an eggcup, so an
acre's plenty to set up some sort of nature reserve. What does your
customer have in a mind - meadow, woodland, marsh, scrub?

Roger