Proportionally Modeling Environments for Unity

Hello, firstly I'm not sure where to post this so I'll post it here.

I am trying to model and environment for unity but I am not sure how to begin the modeling process to make sure that I start it off proportionally correct.

I first want to create a basic environment in Unity with mountains and valley's, but for buildings and other props, I will model in Maya.

-So my question is, how do I start modeling this to make sure that I make everything proportional?
-Should I just start it out and eyeball the whole thing?
-Also, what about the scaling?
-What units should I use within Maya to match those with Unity?

Hope this makes sense, thank you.

 

Unity uses meters.
Make a basic cube in whatever modeling program you use. Make sure the cube is 1m.

Export out of software; and import to unity.

Make a basic cube in unity and place your cube or vertex snap it to the default cube. If it's the same size then scale is set. If not than adjust within your export settings.