Ok, if you are making the ornaments in the link that Moonpi posted up above, only on a 2.5" ball instead of a 3" one, I would change the fabric squares to 2".
I won't bore you with the math, but if you decrease the size of the fabric squares by the same ratio as the decrease in the size of the circumfereence of the balls, you get just a tad over 2". You can make final adjustments in the amount you overlap the triangles as you pin them. (The larger squares would be much too large, imho.)