WPF Layout by Content
The best practice of laying out the controls is to let wpf do the sizing for you as much as you can, so that other controls around it don't need to be resized when you make changes later. This is the concept of Layout by Content. Another example would be if you have Internationalization and the user can switch between different languages that have different lengths and you don't want to calculate the size needed for different languages. Although there will be situations when you need to set a specific size for a control (such as a setting the width of the comboBox wider when the length of all the items in it are too short to look nice), but for the most part letting WPF do the sizing will save you a lot of time.