There is no formula or rule of thumb for the number of nozzles versus impact on robot size for a robotic blast machine. It depends on several factors. In addition to the quantity of nozzles, the payload calculation has to take into account the nozzle size, hose length that's being supported, the nozzle holder design, required robot reach and type of media. All this will add up to more than the weight of the mere nozzle. Another factor to keep in mind is that increase in the number of nozzles also makes the programming complicated due to potential for twisting of hoses as the robot executes its travel path. Another factor to keep in mind is the part geometry/orientation that the nozzles are targeting. Critical / difficult geometries being processed with more than one nozzle could severely limit the motion of the 5th and 6th axes due to blast hose entanglement and twisting.
Whenever we are faced with a situation where we have the need for more than one nozzle, we do a reach study to arrive at the robot model. It is difficult to ascertain robot size without this exercise. Not saying it isn't possible to have multiple nozzles on a robot, but other options to consider would be a nozzle carriage in addition to a robot, or simply a nozzle carriage (if you need multiple nozzles, I can't imagine the part would be intricate enough that it can't be processed with a nozzle carriage.
As a final point, longer reaches will also result in larger robot payload sizes - but this isn't the solution in itself. All the above factors have to be taken into consideration in determining robot size (payload requirements).
Hope that helps.