Can we size the generator just by measuring the current in the utility supply side rather than doing the load calculation?

If load is a static load (no motors involved), then with the help of utility voltage & current, we can size the generaor with 20% safety margin.

But industrial loads consists motors. Hence, load calculation is required to ensure generator is capable to start  highest rated motor without transients.

In any industry, generator size is based upon the highest rated motor & its starting power requirements. 

It depends on the application you are looking at . but for a gas turbine power plant you dont want to do that because the electrical load will change with ambient .

we have to do peak load calculation and also we have to forecast the load ,then we have to go for generator sizing

Load calculation must be done and generator sizing should be 25% more than the total load demand.