Regarding articleship

In a big size firm, you tend to specialize in one area, too early in life. But in a mid size firm, you get a taste of practically everything, which places you in a much better position to decide on your future course of action once you complete your training. Whereas in a smaller firm, you get to learn a lot, there is direct interaction with the person signing the financials, and also a more personal touch.

Depending upon the exposure you get: If you are in a big 4, your future job prospects are  more secure and easy to acquire. If you are in a mid sized firm you have another option of becoming a practitioner whereas in a small size firm, continue with a small firm BUT STUDY VERY HARD AND TRY FOR A RANK. Lots of your sins will be washed away if you manage to secure a rank. And you are free to chose your own future ....;)

And last thing, If your dreams are big and you have ability to achieve it then size of the firm will not be a matter.