There are several considerations when you call or provide a web service that can send a lot of data or take a long time.
In .Net you can set the timeout period to be higher.
Also in .Net... "When the amount of data send or received by the Web service exceeds 4 MB, configure the Web server to handle the larger amount of data."
How to: Enable a Web Service to Send and Receive Large Amounts of Data
How to: Enable a Web Service to Send and Receive Large Amounts of Data
