I have a problem with the legend designer, when I produce an A0 plot the legend does not scale up with the map.
MapInfo is notorious for problems like this. My suggestion would be to export your map to a high resolution jpg or pdf (300dpi should do) and then print from that.
Though, depending on your layers, jpg would probably be the better option as your outputs will be more reliable to what you see on-screen. If 300dpi isn't high resolution enough, bump it up to 600dpi. Though, 300 is good enough for most A0 plots in my experience.