riddle - I am both lock and key - who am I?

Enter! Small or large, as the case may be
I'll assist you - it's easy! - I'm both lock and key

Who am I?

Answer

You are

The CAPS LOCK key

Enter! Small or large, as the case may be

ENTER is another key on the keyboard. CAPS LOCK is used to toggle case between lower and upper, or small and large.

Author's note: in addition, the injunction "Enter! Small or large, as the case may be" can be read as an invitation to enter text into the computer by typing it in, in the appropriate case (lower or upper).

I'll assist you - it's easy!

CAPS LOCK makes it easier to type a string of capital letters, rather than SHIFT.

I'm both lock and key

Lock is in the name, and a key is what you are.