I don't know the exact size, but when I have to drill out something like that the first thing I do is to grind or file down the head of the rivet or pin so I can see the outline of the pin. Then you can measure the pin and also it is a lot easier to centerpunch so you can drill more accurately. If you still can't see the pin outline after filing, put a welding torch on the pin area until it heats up enough to make the outline clear.