RSA is only able to encrypt data to a maximum amount equal to your key size (2048 bits = 256 bytes), minus any padding and header data (11 bytes for PKCS#1 v1.5 padding).
As a result, it is often not possible to encrypt files with RSA directly (and RSA is not designed for this). If you want to encrypt more data, you can do something like:
- Generate a 256-bit random keystring K.
- Encrypt your data with AES-CBC with K.
- Encrypt K with RSA.
- Send both to the other side.
Of course you can also use another key-size, symmetric algorithm and cipher mode - but you get the gist.