Yes, via the normal MUI 'snapshot' feature....

...start the MUI settings interface by selecting 'Settings/MUI'. Select the 'Windows' section and make sure the third of the little system gadget buttons there is activated. After saving the settings every window will have an additional system gadget in the upper right corner. One click on this gadget will snapshot the actual size and position of the window for future sessions. This is a general MUI feature and if you use it on the YAM window you should be able to "snapshot" the window size of YAM permanently.

Note:
If you want to also snapshot the column size of the mail listview have a look at FAQ item #1021