It depends upon the size of IO operations you want to perform to the diskgroup. Just remember ASM will stripe the data on to disks based on the AU size. The default value of 1M is adequate for most of the situations, but you can run the load tests on various units size and see what works best for you.
You have to take in various factors like size of READ/WRITE operations, number of disks in diskgroup etc. I would stick with 1M unless I have strong reason not to do so. I hope this helps.