the largest is Montana's at large district with 1,050,493 and the smallest is Rhode Island 2 with 520,389. I tried to find a full list but couldn't with a few minutes of Googling. I'm a little surprised that Wikipedia doesn't have a page on this.
Perils of the fact that districts are confined to state borders so you've gotta round up or down for some states.
It ranges quite a bit above and below that. (About half a million to a million is the range.)