Generally, you should do whatever works:
1. Since you have real life objects, it makes sense to match their scale (3D modelling apps usually have measurement units set up, meters, foot, etc.). However, when you move away from tanks and into trees and other scenery objects - they probably will need to be custom scaled to fit your game style. An example of this is you modeling a tree and scaling it in the game editor to make it look good (many times you will reuse the same tree, but with a different scale to add variety.)
2. Do not move the camera far away from the worlds origin, but move the world around camera placed near worlds origin. This way, you avoid a lot of precision issues that arise when GPU/physics need to precisely compare values that have poor precision. If you move tank and camera 10km from the origin, your precision will be in a matter of fractions of a millimeter with float32 and can cause effects similar to Z-fighting between transformed models.
3. You will have to tweak global constants to make your game feel good. It is the same way you change worlds relative scale to skip boring parts and pack more interesting things into a small 4x4 km area (town, river, mountain, etc.) that are far away from each other in real world.
4. If they looked bettered scaled - scale them. If they fit as they are - keep that. Games and movies are more of an art that about blind copying reality. So you have found a nice looking tree, that is just modeled 2m tall, of course you scale it and adapt to your game by other means (tweak palette, texture detail, etc.)
5. I doubt there's any difference as long as they take the same amount of pixels on screen to display.