Concrete mixer whose motor got following configuration 5.5 KW 380v 1140 RPM 3 phase what is the suitable size of generator to run this motor?

You should get a 3 phase generator that has 8-12 KW. If you had to you could use a 3 phase 5.5-6 kw but it may wear out faster because you are using 100% power. The 3 phase gen will allow you to select 480 volt which can be converted to 380 for the mixer. The rpm of the motor in irrelevant because the generator is the power source in this situation, once the gen powers the motor it will produce 1140 to power the mixer.