The 7inch tablets would be ideal for people who travel a lot since they are very portable. For these people having a voice calling option on these tablets would ake it more worthwhile.
The 8-inch tablet has a space that is normally at least 30% bigger than a 7-inch tablet. It balances portability and functionality very well. It is a good option for those for whom reading e-books is a worthwhile hobby.
Coming to tablets over 9 inches and above, they are good for those people who spend most of their time indoors. It is a tablet that offers rich gaming experience due to the bigger screen. While it is bulkier than 7 and 8-inch tablets, it is good for editing documents, video conferencing etc.