How to size exhaust louver (plenum) for an exhaust fan 1100 l/s ?

I am agree with Ivan better you go with2m/s to arrive at louver size but keep in mind it will give you louver face size but same will change depending upon your percentage of free area avilable for that louver typicaly that value is50 percent only 

There is not some specific standard which explain exhaust louver sizing, but good engineering practice says that1-2m/s velocity on outlet cross sectional area is enough

generally for fresh air intake we used Sanp Trap louver with maximum 1.5 m/s velocity. for exhaust we can go up to 2.5  to 3.0 m/s for exhaust louver sizing.