sizing

Setting Height to "*" for both RowDefinitions results in both rows growing proportionally:

star sizing

Setting the first RowDefinition to "3*" results in a total of 4 stars for the two RowDefinitions, 3 of which go to the first RowDefinition:

absolute sizing

Absolute sizing uses pixels.

Here, the first RowDefinition is 100 pixels; the other two split the remaining space evenly:

Setting the third row of the grid to "Auto" results in the row using the height of the tallest element in the row (in this case, the orange rectangle):

sizing with attribute syntax

<Grid RowDefinitions="100,*,Auto">