Currently, a Grid's Row/Columns cannot be adjusted from the Properties Panel, and only way within Studio is to use the adorners.
Although the adorners are a great feature, we have seen this be somewhat problematic in terms of discoverability of a critical Grid feature.

(Also, would including the Row/Column definitions in the Properties panel make them potentially be able to be animated?)

See attached image for a proposal on how we can re-use existing Studio interface elements to achieve this.
Note that this solution does not include a dropdown to pick between Star, Pixel, and Auto. In an MVP of this model, the fields would be equivalent to typing values in XAML, so in this simple model, the responsibility to specify the unit of measurement would be on the user. However, I expect this to act as a two-way binding, and any changes to the adorners, would be reflected in the Properties Panel (including unit changes), and vice versa.