This question shows some thought about the consequences of jet impingement on a floating roof.  However, the information is insufficient to do anything more than to generalize.  The obvious answer is to avoid powerful jet impingement on the cover or pontoons.  Aiming the jet mixer horizontally will reduce the force of impingement on the cover.  The nozzle exit diameter is only part of the remaining design question.  The jet velocity is important in estimating the local velocity at a point where the jet may strike the other side of the tank, roof or pontoons.  The distance across the tank will affect the diameter and velocity of the jet at the far side of the tank.  The problem reduces to a conservation of momentum in the jet core and entrained liquid.

The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.