I think you changed the diagram default format through the global menu miscellaneous, this default format applies on the future created diagrams, not on the existing ones
to change the format on an existing diagram you have to open it (double click on it in the browser) then call its menu doing a right mouse click inside it out of any element then sub menu "format" or sub sub menu "landscape formats"
The other problems like too small buttons comes because on your screen a pixel is too small, perhaps the simple way is to not use the max resolution to use Bouml ...