You need to ensure the generator is adequate for the normal power load of the motor, starting load and that there is no excessive voltage drop during running or starting.
I just did a quick Google search and one site was recommending 100 kW (star delta starting) and 225 kW (for direct on line). Another site gave a rule of thumb of 1 kW of generator for each 0.56 to 0.75 kW of motor (80 to 107 kW for a 60 kW motor). Quite a bit of a range here.
Possibly the better way is to calculate the running kW, running kVA, staring kW and starting kVA. Compare these to figures given by the generator manufacturer on their datasheets to select the correct generator. The generator manufacturer can also probably provide some good input on the correct size to use.
Should also be careful about voltage drop. In addition to not stalling the motor, you will want to ensure that any generator control equipment connected to the same bus is not affected.