What image sizes can I print with 80mpx Lumix G9?

rich arent you , makes a clear picture at poster size , why you into making enlargement , 80 meg with an apc is a professional camera .

It will depend on whether you have your own inkjet printer and how large a print it will print. If your printer can only print out 8x11 inch prints then that would be the maximum. Some professional photographers who do their own printing have inkjet priters that can handle much larger prints than 8x11. They are expensive though so if you only print out a few of them then you may as well take your file to a commercial photo processing facility and pay them to print it for you. You may have a camera with lots of small pixels crammed onto a small sensor that is capable of producing large files, but it would not produce large prints that are sharp. There is a reason why professionals use medium format cameras that cost an arm and a leg. They do it because these cameras have large image sensors so they can produce sharp prints that are large sized. 

Because Olympus boxed itself into a corner by betting that its smallish 4/3 sensor can remain competitive, it has pretty much gone out of business by selling its camera division to a private equity firm. IOW, it lost its bet. The world is moving to full frame DSLR and mirrorless cameras, People who buy M4/3 cameras and lenses will soon find themselves with obsolete equipment that are worth very little on the used market. 

If we print at 300 dpi (standard for most printers) then the max print size is 34.5 x 26" (double the x-y dimensions of the standard 20 Mpixel mode.

Of course, you can print at a lower dpi and get a larger print. This works fine if the print is viewed from an appropriate distance (larger prints are viewed further away than smaller prints).

You can print any size you want - unless you're pixel peeping you'll be viewing larger images from a greater distance.  The 80mpx mode in the G9 relies on taking 4 exposures and then merging them - that'll only work if the camera and the subject are completely still.