Question: Is it possible to grow tomatoes as a pot plant? In the limited space I have I can afford pot of maximum depth = 12 cm and diameter = 12 cm. what size pot will be ideal to start with?

In India what would be the best time to plant them and how can I take care of it at later stages?

Sun is not that harsh is winters here - temperature ranges 5-35 degree centigrade, average day temperature would about 24 degree centigrade.

Plant will be placed outdoors and will get all sunlight all through the day.

Answer: 
That's a small pot, and definitely not ideal. I recommend as much larger of a container as you can manage, even if you have to sacrifice drainage to make room. However, you can grow some kinds of tomatoes in small pots (but the harvest may be modest). I've grown Galapagos Island (Solanum cheesmaniae) tomatoes in a foam cup, which produced four fruits indoors before I transplanted them. I've also had fruits on some tomatoes descended from the hybrid variety called Grape. However, there are probably much better-adapted varieties for your purposes.

You'll need to make sure you water it a lot, though, if it's outside (especially if it's hot and dry where you live). It might be worth making a miniature self-watering container. You might try varieties like Micro Tom or Tumbling Tom. I'm not sure if they're heat or drought tolerant, though.

Another thing about small containers is that the plants may need to be fertilized more often.

I'm not sure about India's climate and seasons, personally, but hopefully someone else will answer that.
As an anecdotical answer, here's what happened to a slightly smaller undrained pot Φ~=80mm, h~=90mm. I had intended to transfer it to a larger proper pot but that never happened. The red thing is a 0.5 liter beer can for scale. The height of the plant is H~=500mm and the diameter of the fruit is d~=9mm. It is turning yellow now so I expect it to at very best double in diameter before ripening.

[![enter image description here](https://i.stack.imgur.com/rv7Cd.jpg)](https://i.stack.imgur.com/rv7Cd.jpg)