Audio System Designer Turned IP Camera System Designer (Novice)

I fully understand the placement, positioning and specification required by certain loudspeakers in certain environments to achieve the desired results based on the application. I am applying that same logic to camera placements within various applications, however am battling to understand varying focal lengths. For instance - I am currently designing a solution for a large warehouse - the client would like to monitor employees within the warehouse and proactively protect his assets. The shelves are extremely high (8m) and the width of the aisles are on average 3-4m. The length of the aisle is 25m. I can place the camera at any required height required as the roof beams are at the highest point approx 12m. There are forklifts that pick stock all day. The target height is approx 2m as the client does not need to monitor stock higher up, as it all needs to come down when picked. How do I choose the right camera for these aisles? This is a learning curve for me, so please excuse the simplicity of the question. I am using GeoVision as the requested product.