Scale cube with mouse position on terrain

Hey guys, I am back with a new question.

1. I have got a flat terrain.

2. I have got a Cube-Prefab with the dimensions x=1, y=0.5, z=1.

What I want to achieve:

-The player leftclicks the terrain. The Cube-Prefab gets initiated with one of the corners at the hit.point of the mouse position.

-If the player moves the mouse the Prefab scales in a way, that one corner stays at the original mouse position and the other one follows the present mouse position above the flat terrain. For better understanding I attach an image:

You see that what I want is quite similar to the common drawn rectangle in Rts-Games, but using a prefab in 3-dimensional space on the terrain.

-What it should represent in the game-

I want the player to be able to build rectangular farmland on the terrain and decide its size freely.

Does someone got a hint for me, how you would approche this?