Panel items do not size or position correctly.
Absolute positioning in Motif doesn't mix well with constraint-based positioning, as used by wxWindows to implement left-to-right, top-to-bottom layout.
If you know that all your widgets are going to be positioned and sized explicitly, you should pass the style flag wxABSOLUTE_POSITIONING to the panel constructor, in which case a Motif bulletin board widget will be used instead of a form widget.
Another possibility is to try setting the widget sizes first, and finally setting the panel size.