It depends what you are growing. For tomatoes you need a 5 gallon pot. for chillies you need three gallon pot. For Cilantro/hara dhania and mint/ podina any size will be fine.
It is not good time to start from seeds so buy some starter plants from a nursery.
The ones you buy already potted are meant to be transplanted in bigger pots. If you leave them in their original pots they will not grow. Also when you transplant them to bigger pots you need to make sure that you save them from transplantation shock. That is try to take them out of the pot without exposing the roots. water that them first before trying to take them out. You can watch some you tube videos as to the right methods of transplantation.