REST - WEB API - Design Approach

The scenerio:

The consumer provides a large XML input as a request. I will have to parse the XML and get the results from the Database and provide the XML result as a Response.

Example Input:

< XML >< PARTS >< PART >123123123< / PART >< PART >123123123< / PART >< PART>4535345< /PART>< /PARTS>< / XML>.

Actual input is big. Say for 10,000 parts.

Output i will have to provide the part description, unit of measurement, etc etc by parsing the xml input, find the details from the database and respond back with xml or json.

How can i design REST based WEB API in .NET? What is the best approach?.