Question Problems with some basic stuff

Hi!

I need help with some basic settings/setup to start using Unity. I´ve watched some youtube vids, mostly about creating a "look around" and "move/walk" function. But I realize that I need to get the most basic things straightened out first.

Here are the things I need help with:

1. Messure a size of terrain in meters
2. Ground or lock the terrain so it doesn´t tilt when starting to work with objects, cameras and so on
3. Add a reference height on the terrain plane so I can adjust the scale of textures so it looks natural in game

1. When I create a new terrain it has the value 1000 x 1000 by default. Is that actually 1000 x 1000 meters? If not, how can I determine the messurement unit of the terrain for realistic sizing of the world for my game? I´m guessing that it´s more simple to create my terrain when knowing that I see size and distance in meters so I can relate to a real messurement I know of.

2. When I´ve created a terrain and start to add objects like trees or trying to change the Main Camera position I get the problem that my terrain plane tilts so that the plane doesen´t lie flat anymore. After a while I loose the feeling of where my terrain actually are situated in the Unity Scene editor. Is there a way to ground the terrain plane in some kind of Origo?

3. Let´s say that I´ve created a terrain and painted some ground textures on it and then try it out in Play mode just to see that the texture is way to big to look natural. Is there a way to add a reference of some sort to better know how to scale up or down the textures?

A bonus question:

What´s the difference in hitting the Play button vs the Game button to see how my creation looks like in the "real" game?