Grid Layout

My experience with this is:

1. *
Tells the grid to make this row or column use the remaining space

2. Difference between auto and * ?
Auto sizes the row or column based on the component size within, * just makes the row use whats left  whether it fits the component or not.

3. How is the scale of the width measurement ? what is the total width of the screen? 
Measurement is in pixels.  

4. What does 2* means ? 
If you have two columns (or rows) where one is * and the other is 2*, the second 2* column will always be 2 times wider than the * column.

You actually need 3 rows in that case, and to set their heights to *, *, and 2*. This way, the system will divide the screen in 4 sections, allocate 2 of them to the bottom row, and one each to the other rows.

(You might have to place something in that row, like a transparent image or label, otherwise the system might not display the row in some cases). 

EDIT: sorry, I might have misread what you want - i interpreted your text as "I want half the screen to go to the rows, and the other half to stay blank". If instead you meant "I want each row to have half the screen", just have 2 rows with their heights set to * and *. The system will split the available space in two sections and allocate one to each row.