When creating frames and dialogs, create them with wxDefaultPosition and wxDefaultSize, which will tell WinCE to create them full-screen.
Don't call Fit() and Centre(), so the content sizes to the window rather than fitting the window to the content. (We really need a single API call that will do the right thing on each platform.)
If the screen orientation changes, the windows will automatically be resized so no further action needs to be taken (unless you want to change the layout according to the orientation, which you could detect in idle time, for example). However, if the input panel (SIP) is shown, windows do not yet resize accordingly. This will be implemented soon.