Not sure of the exact size. You may need to measure the ball joints with a caliper if you decide to use the pickle fork. I have used both a pickle fork and a ball joint remover as referenced in the previous post. The latter is a safer bet with regard to preventing tears in the ball joints and it is easier to use in tight spots.
Just for reference there are pickling fork attachments for air hammers if you have a compressor. Just saw one the other day at my local Sears.
JC Whiney have a couple of joint separators at a fraction of the cost of the IAP model. Buy them both and you will be able to separate ball joints and tie rod ends without damaging them.
74 GTV (3 days until it moves to Toronto)
79 Spider - about to get almost all my attention
88 Verde - daily driver