The small eggs I've seen are either 2" or 1-7/8" balls. I have seen both, and they are not interchangeable unless you have a special ball, so I simply have two different inserts, one with each size ball on it. I don't think there's any way you can know for sure which to get now, although if you had to choose, I'd say 2" is more likely.
I agree with those who say to just wait. The other reason is that there are different angles of "drop" on the square part that the ball attaches to, for different heights of trailer/tow vehicle. Again, you won't know till you have the trailer. But, these parts are available at WalMart, so it's reasonable to buy them on site after you have found a trailer, unless you are really in the boonies.