It is very difficult to predict how much you will need to vent. In your case I would guess the vent lines will be in the 6 to 10 mm range, with needle or globe valves installed for regulating. You close the valves as much as possible and open them further when you see that there is a problem with heat transfer. Eventually you will find the right amount of opening to leave them at, but the loss of steam is negligible.
Perhaps I was being a bit pedantic wanting the vents to bypass the condenser. It is very important to keep incondensibles out of total condensers, but you will have a small through flow to the ejector anyway so the incondensibles will not build up.
I agree with Bobby that the vents should be taken from near the condensate outlet at the bottom of the exchanger. Practical experience has shown this to be the best location. I am not convinced that it is because the incondensibles are heavier than steam. In my mind, there is so much turbulence inside the steam chest that the small differences on a molecular level are not going to cause separation. But there is a net flow of steam and condensate towards the condensate outlet and it will entrain the non condensibles in that direction. Whatever the reason, we agree on the location!