Two reasons come to my mind for the first question:

a) The control circuit. Large counter electrode means small impedance, which increases the fraction of the compliance voltage of the potentiostat available for controlling the working electrode.

b) The current distribution. A large counter electrode will produce more homogeneous current distribution on a small working than vice versa.

For the second question, I do not understand your problem. Your small elecytrode is the counter?

thank you Ramon..

My second question is :

I need to use a metal thin film on glass 2mm / 2mm dimension... as working electrode... if i use a platinum wire as counter electrode will there be any problem?

also if you have any idea about how to make this thin film as electrode..i mean how to connect it to a circuit?

a) no problem if you put ir far from the working electrode. Alternatively you can shape the wire to form an spiral of similar size to the working.

b) if your metal deposit is obtained by sputtering you can always make a square with a tail which you will use for electrical contact. Of course, the tail shall be coated by some organic coating to insulate it from the solution.

thank you Ramon..