Proportional Sizing (star sizing)
When a single row’s height or column’s width is set to *, it occupies all the remaining space.
When multiple rows or columns use *, the remaining space is divided equally between them.
Rows and columns can place a coefficient in front of the asterisk (like 2* or 5.5*) to take proportionately more space than other columns using the asterisk notation. A column with width 2* is always twice the width of a column with width * (which is 1*) in the same Grid.
Stretch = 100%
Otherwise you need to use the * star sizing, percentage values are not supported.
Grid cells background colour, padding, borders, etc
Background color and border
Add a Rectangle which stretches to fill the cell with the fill and stroke (border) you want. Or you can use a Border element and set its BorderBrush appropriately (gives better performance). Be sure to add these to the grid before adding any of the other children (or explicitly mark them with the ZIndex attached property), so their Z order puts them behind the main content.
Use autosizing and set the Margin on the child element.