RSA encryption works on messages which are no longer than the public key modulus size (actually the limit is slightly below that, e.g. 117 bytes for a 1024-bit RSA key). The usual method for encrypting things with RSA is to use a hybrid mode: you create a random symmetric key (a bunch of random bytes, with a cryptographically secure random number generator), you encrypt symmetrically your data with that key (using a symmetric cipher such as AES), and you encrypt the symmetric key with RSA. Symmetric ciphers can deal with arbitrary length data. A symmetric key is rather short, typically 16 or 32 bytes, so there is no problem in encrypting that with RSA.
Be warned that you are in the process of designing your own cryptographic protocol, which is a very bad idea. Actually the first and foremost of all cryptography-related bad ideas. Even for something as simple-looking as asymmetric encryption of a blob, people who have tried to design such a protocol have gone through years of multiple versions and security holes, painfully plugged one by one. You should really use an existing protocol for that. One such protocol is OpenPGP. Bouncy Castle is an opensource library which has a C# version, and which includes an OpenPGP implementation. Use it, and you will live longer and happier.