What are the physical upper bounds on the magnitude of an earthquake?
Earthquakes involve slip on faults, most of which occur on plate boundaries. 

Now the width of a fault is usually limited by the downdip limit of the brittle ductile transition. E.g., in case of vertical strike slip faults this limit is between 10-20 km. In subduction zones this limit can be larger as the fault/interface is inclined.

The theoretical length of the fault is limited by the length of the plate boundary. It is impossible to have a fault with a length greater than the circumference of earth.

The final variable is slip. Average slip, based on empirical relationships, doesn't exceed more than 5-15m (rocks can accumulate only so much strain). Though locally it is possible to have slip as high as 50m (e.g., in the case of the Tohoku earthquake.

So for a plate boundary fault which exactly splits the earths brittle upper crust into two halves (in a single earthquake) with an average slip of 15 m, assuming a modulus of rigidity of 30GPa (mean for upper crust) we have:

Moment = Fault_area x rigidity x slip


which for our hypothetical scenario is equal to

= ((2*pi*6371000)*20000)*(30e9)*(15)


or

3.6E23 Nm


which is about Mw9.7.