Use it to help you tune out "false alerts". For example, I have a fox that crosses one of my cameras about every night. I only care about recording humans, so I set the minimum object size setting to be larger than the fox. I don't think I set the maximum object size, but if I had cars driving across the same area and wanted to filter them out, I could set the maximum object size to something larger than a human. Those sizes are 100% custom to my installation, which is probably why Dahua doesn't give any suggestions... it really is going to vary by every install.
I haven't messed with these in awhile (I run
Blue Iris and let it do the motion detection), but I remember using my browser to literally draw a shape for the min object size, and the camera converted that into the numeric values.