sizers are the recommended layout manager for wxWindow/wxPython applications, but sizers are fairly complex. There are many pages about sizers in the wxPython wiki.
PythonCard currently only supports absolute positioning in the resource file. However, you can put sizer code in the openBackground handler, which is similar to how you would use sizers in a wxPython script.
Due to the complexity of sizers, it might be worthwhile to explore additional layout mechanisms and layout types like flow and a simpler grid. At a minimum, if people want to use auto-layout mechanisms they are already familiar with in tkinter, we probably need something equivelant in functionality.
Sizers
If we're going to use sizers, then we need to simplify their usage.
Simon Willison's PythonCardSizerSuggestions
Tkinter Geometry Managers
http://www.pythonware.com/library/tkinter/introduction/index.htm