How is text converted to a number for RSA?
You never use RSA to encrypt large binary objects, since it's to expensive to do many calculations. 

Instead, RSA is used in a key exchange or key transport protocol to send a key for a symmetric-key algorithm, such as AES (with a cipher chaining mode, such as CBC), which is then used to encrypt huge messages.

Also note that textbook RSA isn't secure. Some differences between textbook RSA and deployed RSA are mentioned there: https://crypto.stackexchange.com/a/1449/4874