You initially wanted to detect a person, now you want to detect "small size objects"?

You initially had a goal of 300m to 500m; now you write "a few miles"?

Next, you'll discover that visibility isn't perfect at all times, and this will rule out most choices.

Will you need a photograph (or electronic equiv.) of the intruder?

What about animals and livestock? Do you want an alarm when they enter? Are there any other obstructions such as trees, rocks, terrain?

Your employer is "not satisfied" with a fence; does he plan to build one anyway to help define the perimiter you're monitoring? Will it have helpful attachment points?

Yes, PIR is capable of working in sunlight since it detects moving objects of different temperature than the background. It is difficult to control the range of a PIR, so it's best when aimed inward from the perimeter. PIR with range of 2km? Good luck.