I'm pretty sure that you cannot directly manipulate index extent sizing for indexes. Found this to be true with IDS 7.3, 9.2, and 9.3 so this probably hasn't changed with 9.4 IMHO.
I've asked this question at another BB & I was told that the IDS engine somehow derives the initial/next extent size numbers for indexes from the initial/next extent sizes of the table.
After defragging a DB I have found many large tables with indexes with relatively small initial/next extent sizes for the indexes. Though they are contiguous right now & treated as one extent, they (the indexes) will obviously become fragmented quickly as soon as the DB becomes active.
Make the initial/next extent sizes for the table(s) optimistically large (as you have allowing for 25% growth) and monitor growth.
If there is a way of directly setting index extent sizes, someone please post a solution.