How to predict the magnitude of output voltage in a practical differentiator?
Now my question is how do I predict the magnitude of output
theoretically?

Start by simplifying things. Get rid of the op-amp and picture the capacitor's right-hand terminal shorted to 0 volts/ground: -

Concentrate only on Vin, R1 and C1. What you need to find is the current that flows through R1 and C1. That current is the bit to find. Once you have derived it you can move on to the much simpler task and converting that to an output voltage at the op-amp.
So, calculate the current based on what you know and in the form that you need.