how to find out network traffic for a query in sql server?

I am not sure how you will get it in bytes. But to know the performance eficiency of a query you can use sql server profiler.

To know which queries causes too much traffic
follow ths link,
http://dba.stackexchange.com/questions/756/find-out-what-queries-are-causing-the-biggest-amount-of-network-traffic[^]

To monitor network performance, follow this link

http://social.msdn.microsoft.com/Forums/en-US/adodotnetentityframework/thread/68e33a4b-1b70-4f22-8531-6b86d834ac65/[^]

Another way of calculating the size of data you querying is insert into table and use the sp_spaceused.

For eg.If i have a query

SELECT * FROM employee where YEAR(InsertDate)=2012

I want to know how much data its going to fetch.
For taht i am going to insert these into temp table and use sp_spaceused.

SELECT *
INTO dbo.temptable
FROM employee where YEAR(InsertDate)=2012

exec sp_spaceused temptable

DROP TABLE temptable