So what you need to know about is something called ground sampling distance, or GSD for short. This the distance on ground represented by a single pixel of the camera. The Astro Pi training webinar
says this is 161 meters per pixel.
This was calculated using this GSD calculator tool
and plugging in the values from the camera data sheet
for the V1 camera. Remember you have V2 in your Astro Pi kit, but V1 is on board the ISS. The altitude of the ISS was assumed to be 400 km.
So multiplying the GSD by the horizontal resolution (161 x 2592) gives you about 417 km, so that's the distance on ground between the left and right side of the image.
Have a read of this post
which will give you an idea about how to trigger the camera based on target locations on ground, and you could employ a similar technique to ensure you only take a picture after the ISS travels every 417 km over the ground. You might also want to take into account vertical ground sampling distance for this though.
I hope this helps.