EncryptByKey and VARBINARY Size Question

Hello,

I'm currently learning how to implement Encryption into my SQL database, and I've run into something I don't know how to explain or understand. I'm working on creating a set encryption for a set of columns in a database, and I don't know how to figure out what size the VARBINARY column needs to be. After some trial and error, I've noticed that a clear text of 9 characters needs 68 varbinary characters, 100 cleartext=148 varbinary, and 254 cleartext= 308 varbinary characters. How is the minimum number of varbinary characters calculated? 

Thanks,
Nic Bonzani