1. Small size firm: Never go for a small size firm unless you know personally someone there and can work directly under the partner in the specific area which you like or want to do a career in. For eg, if you plan to do a career in Indirect taxes, it is best to work in a small firm thats only into IDT and then you can gain good exposure from working with the active partners.
2. Middle size firm: This is good for getting average exposure to more than a few areas. The only advantage a middle size firm has over a big4 is that you get exposure to more than one area.
3. Big4 firms: This is the best option coz although there is a demerit that you may be stuck in only one area (stat and tax audit) for your entire articleship, there are several merits such as exposure to professional working environment of top companies (most top companies have big4 firms as their auditors), in depth analysis of working and business processes of companies, contacts with top management helps in good networking. The whole experience is way different from mid sized firms.
But it is a big struggle coz you wont get more than 4 months leave for CA exams and you will have to give up your social life if you want to get the best out of the big4 experience. It is definitely worth it. Later on, a big4 articleship on your CV and the experience it gives places you MILES ahead of your other friends and classmates.