I always heard that you needed an active congregation average of 350 to split to a new ward. So I imagine an average congregation size would be in the 150-250 range.
On my mission, on the far western part of the United States it wasn't uncommon to see wards in the 60s and 70s numbers of people with little to no leadership.
Let's see: 14,000,000 divided by 25,000 units is about 560 per "average" congregation.
I live in a Texas stake with a total of 3 wards and 6 branches. Each ward has attendance around 110, and the 6 branches average out to about 40 each, at the most. Our stake boundaries include about 120,000 square miles (I'm serious) with an average total weekly attendance among the NINE units of around 570/week (close to the worldwide average membership of ONE unit).