I have to encrypt XML file and send in network to place in a private server.
My other application will decrypt and import to SQL the file by giving the actual Public key.
Could you please suggest me best way for this.
File: XML Type
Possible size: Up to 250 MB
Technology: C#.NET 2008
Use XML encryption. It is supported directly in .NET through the EncryptedXml class.
There is an example on MSDN here: How to: Encrypt XML Elements with Asymmetric Keys.
Note that you should use the public key to encrypt and the private key to decrypt (and not the other way around as you wrote in the question).