I beleive It is possible to mix the size of the disk but really is not recommended:
* The first problem you will have is you will probably have to destroy the aggregate and start again with removing this number of disks.
* Generally, differant disk types are not allowed within a single aggregate. i.e SAS and SATA mix.
* ONTAP will write size the disk so you would won't get any benefit from mixing disk sizes if creating a single aggregate. 1Tb disk will be sized to 300Gb
* You should not mix disk speeds (10k & 15k) in the same aggregate. It is possible but will limit the performance on the faster drives.
In essence, it's best to purchase a new shelf with the same disks if you are creating a single aggregate other you will have perfromance issues. This is my best understanding of how it works but maybe a guru on the forum can give you exact details.