Before we got our 2 Deere 9750 STS combines last year we ran a Deere 9610 and a 9500. Always ran a 8 row corn head, and 30ft grain heads. You could run a 25ft, but the 9610 is more then able to handle a 30ft.
go for a 30 foot head. Anything less and you will not be able to keep the combine running at full capacity, and then you will be disappointed with your threshing performance. That machine works best with a full load.