Our ball inflates to a little over 2' in diameter, but I think that's because on the Sportogo website (
http://www.sportogo.com/2565cmbladder.aspx) they sell it as a 25" ball.
What I'm wondering is if FIRST was intending for the balls to be 24" and made the fabric a bit smaller than the ball, which might have prompted the vague inflation instructions about just pumping until the fabric is taunt but not stretched. Either way it seems like a PSI value would have been much easier to implement instead of having to make a specific segment on ball inflation, unless they wanted teams to be able to handle game pieces that slightly varied.