Canon published an article that they are exploring the Large Sensor Image Future. They shows a 20-cm-square large image sensor is the largest size that can be manufactured based on these dimensions, and is equivalent to nearly 40 times the size of a 35 mm full-frame CMOS sensor.
According to recently rumors on CR, the upcoming Canon EOS 5DS/5DS R Mark II will not see a big bump in resolution. The Canon EOS 5DS/R Mark II will be announced in the second half of 2018, likely at Photokina 2018.
Just a quick update from CR, it is confirmed that Canon EOS 6D Mark II will have a brand new 26MP full frame CMOS sensor. As rumored before, EOS 6D Mark II will be officially announced on June 29, 2017, shipping in early August.