Scale and dimensions problem importing to Unity I'm trying to import a simple plane mesh into Unity (to generate some map)

I have my simple plane that I want scale to be 1x1x1 and dimensions 50x50 but whatever I do; change dimensions scale changes i cannot make it align perfectly in Unity.

If i can align one edge; the other is not perfectly aligned.
I'm exporting as .blend file.

I've tried the following answer :How to align a cluster of points?

But i can't make all my points 'z to be 0; why ?
What am i missing here ?
From your question your trying to do 3 things at once. maybe some of these tips will help.

To keep the scale at 1,1,1  make your changes in Edit Mode, not Object Mode.

If your object is not scaled to 1,1,1 you can reset this with:
'Apply Scale'. 'ctrl+A -> scale'

To make your plane flat, select all the vertices and scale them on the Z to zero. 

Same to align any row of vertices, select the row, and scale X to 0.

To put the plane at the Global Position z=0, Set your 3d cursor, and then snap the plane to it.