[chris (OP)]

What features on the Earth's surface can astronauts on the International Space Station (ISS) see through their windows?

Specifically, how big does something have to be to be visible from the ISS?

[RD]

Angular-resolution of unaided human eye is ~0.02°
The altitude of ISS is ~400km , so astronauts  could just see an object ~140m across on Earth as a point,
 (ignoring atmospheric blurring)   

Here's a photograph of the pyramids from the ISS , which are ~200m across, (bottom right corner) ... 

https://www.nasa.gov/content/egyptian-pyramids-seen-from-station/

But that's probably using a telephoto-lens, rather than the unaided-eye.

[evan_au]

Astronauts can see lightning, which is a few centimeters wide - but extremely bright.
Of course, a lightning stroke can be a kilometer or more long - and light up clouds that are a kilometer or more across....

[Janus]

The average resolution of the human eye is 1 arc minute (for 20/20 vision).  This means that objects would have to be 1/60 of a degree apart in your visual field for you to distinguish them as being separate objects.   For the ISS, which orbits at ~400 km above the ground, this means that objects on the ground would have to be 116 meters apart to be resolved as separate objects. So 116 meters is about the smallest object you could see from the ISS. (Unless you had better than 20/20 vision. Even then, the best possible human eyesight would require something to be ~47 meters across to be seen as a distinct object.)

An Eagle however, with ~8 times the visual resolution of the average human, would be able to distinguish objects as small as 14.5 meters across.

[Bored chemist]

It's also important to recognise that you need contrast to see things. A 200 meter white blob might be visible on a black background, but not on a white one.

Also, I'm told you can see a candle flame from  30 miles, though that's debatable
https://gizmodo.com/scientists-calculate-from-just-how-far-you-can-see-a-ca-1721269534
Allowing for the inverse square law  you should be able to see a group of 64 candles (the ISS is about 8 times that distance) if the 30 mile figure is right and about 25000 if the  figure of 1.6 miles is correct.

So, for example, a camp fire in a remote desert should be visible (at night).

[syhprum]

I would imaging was large airport might just about be recognisable if the lighting was right

[Bored chemist]

I would imaging was large airport might just about be recognisable if the lighting was right

One feature of many airports is that they are in open spaces, and brightly lit.
There are pictures...
http://www.thisisinsider.com/airport-photos-from-space-iss-2017-4