Articles related doubt

I have a couple of doubts so kindly help me with them.

1)When to register for articles? After interviewing with CA or before?

2)How to know whether a firm is mid size or smal firm.

3)How to get an appointment with a particular firm for interview; phone or email?

4)Is there any website or other service where I can find firms in particular area?
if not it would be great if you know some mid size firm in or near Thane.