Imagine you're in a pinhole camera, or better yet, picture a large room that's been made into a camera obscura with 1 small diameter hole in the window looking outside, and a large flat wall opposite that.

Ok, now imagine what you'd see through that hole if you were looking out from various points on the wall. Putting your head near the floor for instance, you'd be looking up through the hole and out onto perhaps the top of a tree, the sky, or the roof of your neighbors house. Now put your head near the ceilling and through the hole you can only see the ground outside, the trunk of a tree, or the sidewalk.

The aperture only lets in a small circle of light from any given direction and this shines on the back wall. Imagine every point on the back wall and knowing that there's a ray of light making a beeline from the outside to the inside... well you can see basically how the image is formed.

And it's true, you could have a 5-foot diameter window and as long as the "film plane" was (perhaps) several hundred feet on the other end, an image would be formed.